In women, the response to FSH varies depending on the FSH receptor (FSHR) genotype, as determined by the non-synonymous SNP rs6166, which exchanges the amino acid Asn to Ser in codon 680. This SNP is very common, with a minor allele frequency of 0.4. Women homozygous for Ser at amino acid position 680 of the FSHR are less sensitive to endogenous and exogenous FSH compared to those homozygous for Asn and require more FSH for multiple follicular growth and maturation in assisted reproduction. The investigators hypothesize that the variable response to FSH in unselected infertile men is due to a different individual sensitivity to FSH as determined by the common FSHR polymorphism rs6166. In particular the investigators will test the hypothesis that men homozygous for Asn at 680 (wild type) will respond better to exogenous FSH treatment in terms of sperm DFI compared to men homozygous for Ser, assessing sperm DFI as pharmacodynamic parameter of FSH.