The identification of genes encoding complex traits has traditionally been achieved by
genome-wide scanning and the candidate gene approach, but these methods do not
constitute a reliable strategy for the systematic exploration of a genetic network that
causes phenotypic variation in complex traits1. Protein networks provide a systems-level overview of genetic
organization and enable the functional modules underlying complex traits to be
dissected, which facilitates the prediction of novel candidate genes for a trait2. In cattle, some approaches related to
interaction networks have been utilized to identify candidate genes related to
phenotypic differences such as marbling3, genes involved in estrus (behavior) in dairy cattle4, and single nucleotide polymorphisms
(SNPs) associated with growth traits in Mexican Charolais cows5.
-
1
Reproductive genomics in domestic animals, 2010
-
2
WheatNet: A genome-scale functional network for hexaploid bread
wheat, Triticum aestivum
Mol Plant, 2017
-
3
Identification of candidate genes related to bovine marbling
using protein-protein interaction networks
Int J Biol Sci, 2011
-
4
Prioritization of candidate genes for cattle reproductive traits,
based on protein-protein interactions, gene expression, and
text-mining
Physiol Genomics, 2013
-
5
Associations of SNPs located at candidate genes to bovine growth
traits, prioritized with an interaction networks construction
approach
BMC Genet, 2015
Reproduction is an essential element of livestock production, and fertility traits are of
particularly significant economic importance; it is a very complex process that involves
numerous consecutive events, including gametogenesis, fertilization, and early embryo
development, that must be accomplished in a well-orchestrated manner to achieve a
successful pregnancy6.
-
6
Unravelling the genomic architecture of bull fertility in
Holstein cattle
BMC Genet, 2016
An improved understanding of the mechanisms that control fertility traits at the organ,
cellular, and molecular levels could aid the development of strategies to improve and/or
monitor fertility4. The objective of
this work was to conduct a search guided by a functional interaction network to identify
key genes controlling reproductive traits in cattle and explore genetic variation in
those identified genes with potential to be associated with reproductive traits.
-
4
Prioritization of candidate genes for cattle reproductive traits,
based on protein-protein interactions, gene expression, and
text-mining
Physiol Genomics, 2013
A literature review was conducted, and the Genie software
(http://cbdm-01.zdv.uni-mainz.de/~jfontain/cms/?page_id=6) was used to perform
PubMed-based text mining of genes that had been previously associated with bovine
reproductive traits (reference genes). To identify and prioritize candidate genes for
the functional network, the interactions of the reference genes were extracted, and the
degree of association with reproduction (DAR) was calculated for each of the genes in
the subnet as follows:
D
A
R
=
Σ
j
∈
r
e
f
g
e
n
e
s
W
i
j
.
Σ
j
∈
r
e
f
g
e
n
e
s
P
i
j
Where Wij is the weight of the linkage connecting protein i
and reference protein j, and Pij is the number of links
connecting protein i and reference protein j
(excluding itself). Thus, the probability that each of these proteins is associated with
reproduction was evaluated based on their interactions with genes whose biological
function had already been associated with this trait5. To select the candidate genes associated with phenotypic
variations in reproductive traits, the DAR score was used to calculate the positive
predictive value (PPV), which indicates the likelihood that a gene is associated with
reproduction, so the selection criterion was the highest value of PPV obtained in this
analysis, i.e., 0.35,7.
-
5
Associations of SNPs located at candidate genes to bovine growth
traits, prioritized with an interaction networks construction
approach
BMC Genet, 2015
-
5
Associations of SNPs located at candidate genes to bovine growth
traits, prioritized with an interaction networks construction
approach
BMC Genet, 2015
-
7
Association of SNPs in dopamine and serotonin pathway genes and
their interacting genes with temperament traits in Charolais
cows
J Appl Genet, 2016
From candidate genes, the Ubiquitin B (Ubb) gene was selected as a
target. The genetic variation in the Ubb gene was investigated using
eleven DNA samples from four different cattle breeds (3 Holstein, 2 Charolais, 3 Brahman
and 3 Angus). Primers UBB-F 5’-GAGAGATTTGTGAGAGATCTTGACG-3’ and UBB-R
5’-CCATTTTAACCTGTTGAGTACCCA-3’ were designed to cover and resequence the bovine
Ubb gene (GenBank accession number AC_000176.1). The resulting PCR
fragments were purified using Exo-SAP-it (Thermo Fisher Scientific, Waltham, MA, USA),
and bidirectional sequencing was achieved using the BigDye® Terminator procedure and an
ABI PRISM 3100 Genetic Analyzer DNA sequencer (Applied Biosystems, Foster City, CA,
USA). Sequences were aligned with ClustalX 2.0.88.
-
8
Clustal W and Clustal X version 2.0
Bioinformatics, 2007
The presence of SNPs in the resulting sequences was determined by visual inspection of
the sequence chromatograms, and the SNPs were defined according to their presence in the
screening population associated with the three expected genotypes.
Amplification-created restriction sites coupled to PCR (PCR-ACRS) were designed to
genotype the nonsynonymous SNP rs110366695 identified in the previous sequencing
screening, and following PCR, the fragments were digested using 2.5 U of
Hinf I enzyme and analyzed on a 2.5% agarose gel. The following
digestion patterns were observed: 210+132+130+18 bp (allele G) and 210+155+150 bp
(allele T).
A population of sixty-seven young Angus and Charolais bulls were genotyped with PCR-ACRS.
The allelic and genotypic frequencies were calculated for each breed, and deviations
from Hardy-Weinberg equilibrium were tested by running GENEPOP statistical package
version 4.29.
-
9
GENEPOP (Version 1.2): Population genetics software for exact
tests and ecumenicism
J Heredity, 1995
A set of 385 reference genes associated with reproductive traits in cattle, through SNPs,
expression profiles or their biological function, were identified. According to the PPV,
the genes that presented a DAR≥11 had a greater than 33% probability of being associated
with reproductive traits in cattle, and those meeting this criterion belonged to the
ubiquitin family: Ubiquitin C (Ubc; Gene ID: 444874) and B
(Ubb; Gene ID: 281370).
The importance of these proteins in the topology of the interaction network was
determined according to the number of interactions; in this case, Ubb
and Ubc have 3,775 interactions of 3,856 possible, so they are very
important hubs. According to BiNGO, a Biological Network Gene Ontology tool
(https://www.psb.ugent.be/cbd/papers/BiNGO), in the subnet that forms
Ubb and Ubc, the annotation of Gene Ontology
51094, "positive regulation of developmental process", is overrepresented with
a p-value of 4.8 E-09. This result makes sense and relates to reproduction in cattle, as
this term of biological process refers to any process that activates or increases the
rate or extent of development and whose specific outcome is the progression of an
organism over time from an initial condition (e.g., a zygote, or a young adult) to a
later condition (e.g., a multicellular animal or an aged adult). Figure 1 depicts the Ubb and Ubc
interaction with 23 reference gene modules, i.e., genes previously associated with
reproduction in cattle8-33.
-
8
Clustal W and Clustal X version 2.0
Bioinformatics, 2007
-
33
A single nucleotide polymorphism-derived regulatory gene network
underlying puberty in 2 tropical breeds of beef cattle
J Anim Sci, 2011
Thumbnail

A) Heifer conception rate, SNPs related to interval to insemination. B)
Effects of the Well of the Well (WOW) system and embryo density on developmental
rates, genes differentially regulated in embryos cultured in vitro. C) Daughter
pregnancy rate, heifer conception rate, cow conception rate. D) Term survival in
embryos of differentially regulated genes, potential of pretransfer endometrial
and embryo gene expression patterns E) Blocking apoptosis in bovine embryos,
gene differentially regulated embryos treated with CSF2. F) Immune function and
developmental genes expressed in the endometrium, endometrial genes
differentially regulated in lactating cows. G) Genes differentially regulated in
oocytes compared to 8-cell embryos, global activation of the embryonic genome.
H) Cow conception rate. I) Igf1 acts in thermoprotection on
bovine embryos, genes differentially regulated in embryos treated with
Igf1. J) Endometrial genes differentially regulated in
pregnant cows and associations with fertility of lactating dairy cows. K) Genes
differentially regulated in oocytes compared to blastocysts, candidate genes for
the characterization of the development. L) Liver genes differentially regulated
during the transition period, determination of hepatic adaptations occurring
from late pregnancy. M) Genes differentially regulated in the oviduct of cows at
diestrus compared to estrus. N) Genes in cumulus cells regulated by the LH
surge, cumulus cells in development and fertility of oocytes. O) Genes
differentially regulated at different stages of oocyte maturation. P) Estimated
relative conception rate, net merit, and fat yield. Q) Calving rate (beef
cattle), net merit, fat percent, and productive life. R) SNPs related to
interval to insemination. S) Embryo development on the blastocyst stage. T)
Differential regulation in cumulus cells from in vivo embryos
compared to in vitro embryos. U) Antiapoptotic in embryos
improves embryo competence. V) Mammary genes differentially regulated during
lactation.
Figure 1
Ubb and Ubc interaction network
The ubiquitin protein (Ub) is widespread in all eukaryotic cells, it has a conserved
structure which has been interpreted as an indication of its important role in cell
metabolism. Through the process of ubiquitination, Ub lead protein degradation and
regulate a different biological events including cell cycle progression,
membrane-receptor endocytosis, antigen occurrence in the immune system, and even
retroviral infection34. Ubiquitination
is achieved through the covalent binding of 76-AA, 8.5 kDa ubiquitin to the ε-amino
group on the Lys-residues of the substrate via the C-terminal AA residuum of ubiquitin
(G76). This process requires ATP hydrolysis and a set of ubiquitin-conjugating factors
including ubiquitin-activating (UBA) and conjugating (Ubc) enzymes11.
-
34
The possible biological and reproductive functions of
ubiquitin
Hum Reprod, 2001
-
11
Effect of the microenvironment and embryo density on
developmental characteristics and gene expression profile of bovine
preimplantative embryos cultured in vitro
Reproduction, 2009
Among multiple functions of Ub system, those involved in developmental and reproductive
processes are relevant. In the former case, there are studies in different models
including developmental transitions in Dictyostelium discoideum and
development specificity in C. elegans35,36. In chicks it has been involved in embryogenesis, and also in
the human myogenesis and brain development34. At reproductive processes level, human Ub has been reported
as the main protein in seminal plasma and the ubiquitination system has been implicated
with fertility problems in humans and other species including cattle37,38. It has been reported that a high proportion of
ubiquitinated spermatozoa in the ejaculates of different species is related to
infertility38.
-
35
A ubiquitin-conjuganting enzyme is essential for developmental
transitions in Dictiostelium
Mol Biol Cell, 1997
-
36
An essential ubiquitin conjugating enzyme with tissue and
developmental specificity in the nematode C. elegans
EMBO J, 1996
-
34
The possible biological and reproductive functions of
ubiquitin
Hum Reprod, 2001
-
37
Sperm ubiquitination positively correlates to normal morphology
in human semen
Hum Reprod, 2005
-
38
Ubiquitin as an objective marker of semen quality and fertility in
bulls, 2004
-
38
Ubiquitin as an objective marker of semen quality and fertility in
bulls, 2004
In cattle, increased ubiquitin levels have been associated with both increased levels of
damage to sperm DNA and with reduced fertility39. A negative correlation between sperm ubiquitin and sperm
count, swirl and % normal morphology has been reported in bulls and evidence that
increased ubiquitin levels in bull sperm are predictive of both poor semen quality and
fertility has been also found 39. This
evidence has allowed the use of ubiquitinated spermatozoa as an useful tool to identify
fertility problem40,41.
-
39
Percentage of ubiquinated spermatozoa does not correlate with
fertilizing capacity of thawed bovine semen
Reprod Dom Anim, 2013
-
39
Percentage of ubiquinated spermatozoa does not correlate with
fertilizing capacity of thawed bovine semen
Reprod Dom Anim, 2013
-
40
Ubiquitin-based sperm assay for the diagnosis of male factor
infertility
Hum Reprod, 2001
-
41
Increased levels of sperm ubiquitin correlate with semen quality
in men from an andrology laboratory clinic population
Hum Reprod, 2004
Even though there is some evidence that the biological mechanisms through ubiquitination
systems affect the different species fertility, the process of spermatozoon ubiquitin
tagging and the role of this process in sperm biology remains unclarified.
Searching for additional evidence to consider these genes as candidate genes, molecular
characterization was achieved on the Ubb gene. According to the NCBI
database, the length of the gene is 1898 bp, and it contains one exon at position 841 to
1758. In this database, 19 and 15 SNPs have been reported in the coding and noncoding
sequences, respectively, and the amplified 1328-bp fragment enabled the identification
of 5 SNPs in the study population, 3 (rs109592218, rs110007734 and rs110366695) in the
coding region and 2 (rs720990890 and rs439271103) in the noncoding region. The
transversion rs110366695 (G/T) located at exon 1 is particularly interesting because it
causes a nonsynonymous functional change, and the GAG codon that produces glutamic acid
(Glu, E) changes to UAG, which is a stop codon, thus predicting a truncated protein that
is 287 amino acids shorter than the nonmutated protein. Figure 1 shows the allelic frequencies of SNP rs110366695 evaluated in the
Angus and Charolais beef cattle breeds; allele G showed the highest frequencies (0.542
and 0.750, respectively). Significant departures (P<0.001) from
Hardy-Weinberg equilibrium were identified for the Angus breed, and lower-than-expected
numbers of heterozygotes were found for these loci.
To the current knowledge, there have been no previous molecular studies aimed at
evaluating the effects of Ubb genetic variation on semen quality
despite the demonstrated physiological importance of the Ubb gene. The
obtained results support the Ubb gene as a strong candidate gene with
genetic variations to be tested for association with reproductive traits. Unfortunately,
in Mexico, phenotyping for reproductive traits is not a common practice, and additional
efforts must be made to prepare a wide database that allows confirmation of its genetic
influence on these traits, particularly of the transversion rs110366695 (G/T).
An analysis-based protein-protein interaction network has been previously validated as a
useful tool for identifying causal genes associated with economic traits in bovines and
other species. The obtained results provide information about the potential of
Ubb and Ubc as candidate genes for reproductive
traits, particularly semen quality, and justify further research aimed at exploring both
the effects of protein truncation and its potential as a molecular marker.
Acknowledgments
The authors acknowledge the financial support received from the research grant
project CONACYT 294826 and SIP 20195072.
Literature cited
-
1
Jiang Z, Ott TL. Reproductive genomics in domestic animals. Iowa,
USA: Wiley-Blackwell; 2010.
Links
-
2
Lee T, Hwang S, Kim CY, Shim H, Kim H, Ronald P,
et
al. WheatNet: A genome-scale functional network for hexaploid bread
wheat,
Triticum aestivum. Mol Plant
2017;(8):1133-1136.
Links
-
3
Lim D, Kim NK, Park HS, Lee SH, Cho YM, Oh SJ,
et
al. Identification of candidate genes related to bovine marbling
using protein-protein interaction networks. Int J Biol Sci
2011;(7):992-1002.
Links
-
4
Hulsegge I, Woelders H, Smits M, Schokker D, Jiang L, Sørensen P.
Prioritization of candidate genes for cattle reproductive traits, based on
protein-protein interactions, gene expression, and text-mining. Physiol Genomics
2013;(10):400-406.
Links
-
5
Paredes-Sánchez FA, Sifuentes-Rincón AM, Segura CA, García PCA,
Parra BGM, Ambriz MP. Associations of SNPs located at candidate genes to bovine
growth traits, prioritized with an interaction networks construction approach.
BMC Genet 2015;(91):1-12.
Links
-
6
- Han Y, Peñagaricano F. Unravelling the genomic architecture of
bull fertility in Holstein cattle. BMC Genet 2016;(1):143.
Links
-
7
Garza-Brenner E, Sifuentes-Rincón AM, Randel RD, Paredes-Sánchez
FA, Parra-Bracamonte GM, et al. Association of SNPs in dopamine and serotonin
pathway genes and their interacting genes with temperament traits in Charolais
cows. J Appl Genet 2016;(3):1-9.
Links
-
8
Larkin MA, Blackshields G, Brown NP, Chenna R, McGettigan PA,
McWilliam H,
et al. Clustal W and Clustal X version 2.0.
Bioinformatics 2007;(21):2947-2848.
Links
-
9
Raymond M, Rousset F. GENEPOP (Version 1.2): Population genetics
software for exact tests and ecumenicism. J Heredity
1995;(3):248-249.
Links
-
10
- Cory AT, Price CA, Lefebvre R, Palin MF. Identification of
single nucleotide polymorphisms in the bovine follicle-stimulating hormone
receptor and effects of genotypes on superovulatory response traits. Anim Genet
2013;(2):197-201.
Links
-
11
- Hoelker M, Rings F, Lund Q, Ghanem N, Phatsara C, Griese J.
Effect of the microenvironment and embryo density on developmental
characteristics and gene expression profile of bovine preimplantative embryos
cultured in vitro. Reproduction 2009;(3):415-425.
Links
-
12
- Cochran SD, Cole JB, Null DJ, Hansen PJ. Discovery of single
nucleotide polymorphisms in candidate genes associated with fertility and
production traits in Holstein cattle. BMC Genet
2013;(49):14-49.
Links
-
13
- Fair T, Carter F, Park S, Evans AC, Lonergan P. Global gene
expression analysis during bovine oocyte
in vitro maturation.
Theriogenology 2007;(68):91-97.
Links
-
14
- Salilew WD, Hölker M, Rings F, Ghanem N, Ulas-Cinar M, Peippo
J,
et al. Bovine pretransfer endometrium and embryo
transcriptome fingerprints as predictors of pregnancy success after embryo
transfer. Physiol Genomics 2010;(2):201-218.
Links
-
15
- Loureiro B, Oliveira LJ, Favoreto MG, Hansen PJ.
Colony-stimulating factor 2 inhibits induction of apoptosis in the bovine
preimplantation embryo. Am J Reprod Immunol 2011;(65):578-588.
Links
-
16
- Cerri RL, Thompson IM, Kim IH, Ealy AD, Hansen PJ, Staples CR,
et al. Effects of lactation and pregnancy on gene
expression of endometrium of Holstein cows at day 17 of the estrous cycle or
pregnancy. J Dairy Sci 2012;(10):5657-575.
Links
-
17
- Fayad T, Lévesque V, Sirois J, Silversides DW, Lussier JG.
Gene expression profiling of differentially expressed genes in granulosa cells
of bovine dominant follicles using suppression subtractive hybridization. Biol
Reprod 2004;(2):523-533.
Links
-
18
- Misirlioglu M, Page GP, Sagirkaya H, Kaya A, Parrish JJ, First
NL,
et al. Dynamics of global transcriptome in bovine matured
oocytes and preimplantation embryos. Proc Natl Acad Sci
2006;(50):18905-18910.
Links
-
19
- Cochran SD, Cole JB, Null DJ, Hansen PJ. Single nucleotide
polymorphisms in candidate genes associated with fertilizing ability of sperm
and subsequent embryonic development in cattle. Biol Reprod
2013;(3):1-7.
Links
-
20
- Bonilla AQ, Oliveira LJ, Ozawa M, Newsom EM, Lucy MC, Hansen
PJ. Developmental changes in thermoprotective actions of insulin-like growth
factor-1 on the preimplantation bovine embryo. Mol Cell Endocrinol
2011;(1-2):170-179.
Links
-
21
- Mamo S, Sargent CA, Affara NA, Tesfaye D, El-Halawany N,
Wimmers K, Gilles M, Schellander K, Ponsuksili S. Transcript profiles of some
developmentally important genes detected in bovine oocytes and in vitro-produced
blastocysts using RNA amplification and cDNA microarrays. Reprod Domest Anim
2006;(6):527-534.
Links
-
22
- Graber M, Kohler S, Kaufmann T, Doherr MG, Bruckmaier RM, van
Dorland HA. A field study on characteristics and diversity of gene expression in
the liver of dairy cows during the transition period. J Dairy Sci
2010;(1):5200-5215.
Links
-
23
- Bauersachs S, Rehfeld S, Ulbrich SE, Mallok S, Prelle K,
Wenigerkind H,
et al. Monitoring gene expression changes in
bovine oviduct epithelial cells during the oestrous cycle. J Mol Endocrinol
2004;(2):449-466.
Links
-
24
- Assidi M, Dieleman SJ, Sirard MA. Cumulus cell gene expression
following the LH surge in bovine preovulatory follicles: potential early markers
of oocyte competence. Reproduction 2010;(6):835-852.
Links
-
25
- Salhab M, Tosca L, Cabau C, Papillier P, Perreau C, Dupont J,
et al. Kinetics of gene expression and signaling in bovine
cumulus cells throughout IVM in different mediums in relation to oocyte
developmental competence, cumulus apoptosis and progesterone secretion.
Theriogenology 2011;(1):90-104.
Links
-
26
- Khatib H, Monson RL, Huang W, Khatib R, Schutzkus V, Khateeb
H, et al. Short communication: Validation of in vitro fertility genes in a
Holstein bull population. J Dairy Sci 2010;(93):2244-2249.
Links
-
27
- Rosenkrans Jr C, Banks A, Reiter S, Looper M. Calving traits
of crossbred Brahman cows are associated with Heat Shock Protein 70 genetic
polymorphisms. Anim Reprod Sci 2010;(3-4):178-182.
Links
-
28
- Pimentel EC, Bauersachs S, Tietze M, Simianer H, Tetens J,
Thaller G,
et al. Exploration of relationships between
production and fertility traits in dairy cattle via association studies of SNPs
within candidate genes derived by expression profiling. Anim Genet
2011;(3):251-262.
Links
-
29
- Gad A, Besenfelder U, Rings F, Ghanem N, Salilew-Wondim D,
Hossain MM, et al. Effect of reproductive tract environment following controlled
ovarian hyperstimulation treatment on embryo development and global
transcriptome profile of blastocysts: implications for animal breeding and human
assisted reproduction. Hum Reprod 2011;(7):1693-1707.
Links
-
30
- Tesfaye D, Worku D, Rings F, Phatsara C, Tholen E, Schellander
K,
et al. Identification and expression profiling of microRNAs
during bovine oocyte maturation using heterologous approach. Mol Reprod Dev
2009;(7):665-677.
Links
-
31
- Jousan FD, Hansen PJ. Insulin-like growth factor-I promotes
resistance of bovine preimplantation embryos to heat shock through actions
independent of its anti-apoptotic actions requiring PI3K signaling. Mol Reprod
Dev 2007;(2):189-196.
Links
-
32
- Mani O, Körner M, Sorensen MT, Sejrsen K, Wotzkow C, Ontsouka
CE,
et al. Expression, localization, and functional model of
cholesterol transporters in lactating and nonlactating mammary tissues of
murine, bovine, and human origin. Am J Physiol Regul Integr Comp Physiol
2010;(2):642-654.
Links
-
33
- Fortes MR, Reverter A, Nagaraj SH, Zhang Y, Jonsson NN, Barris
W,
et al. A single nucleotide polymorphism-derived regulatory
gene network underlying puberty in 2 tropical breeds of beef cattle. J Anim Sci.
2011;(6):1669-1683.
Links
-
34
- Bebington C, Doherty FJ, Fleming SD. The possible biological
and reproductive functions of ubiquitin. Hum Reprod Update
2001;(1):102-111.
Links
-
35
- Clark A, Nomura A, Mohanty S, Firtel RA. A
ubiquitin-conjuganting enzyme is essential for developmental transitions in
Dictiostelium. Mol Biol Cell
1997;(8):1989-2002.
Links
-
36
-Zhen M, Schein JE, Baille DL, Peter E, Candido M. An essential
ubiquitin conjugating enzyme with tissue and developmental specificity in the
nematode
C. elegans. EMBO J
1996;(15):3229-3237
Links
-
37
-Muratori M, Marchiani S, Forti G, Baldi E. Sperm ubiquitination
positively correlates to normal morphology in human semen. Hum Reprod
2005;(20):1035-1043.
Links
-
38
- Sutovsky P, Geary T, Baska KM, Manandhar G, Feng D, Lovercamp
KW, Sutovsky M. Ubiquitin as an objective marker of semen quality and fertility
in bulls. Proc Nebraska Appl Reprod Strat in Beef Cattle
2004;185-199.
Links
-
39
- Rodríguez-Lozano I, Ávalos-Rodríguez A, Castillo-Juárez H,
Borderas-Tordesillas F, Roa-Vidal JJ, Rosales-Torres AM. Percentage of
ubiquinated spermatozoa does not correlate with fertilizing capacity of thawed
bovine semen. Reprod Dom Anim 2013;(1):27-31.
Links
-
40
- Sutovsky P, Terrada Y, Schatten G. Ubiquitin-based sperm assay
for the diagnosis of male factor infertility. Hum Reprod
2001;(2):250-258.
Links
-
41
- Sutovsky P, Hauser R, Sutovsky M. Increased levels of sperm
ubiquitin correlate with semen quality in men from an andrology laboratory
clinic population. Hum Reprod 2004;(3):628-638.
Links