It is believed that genetic defects make an important contribution to male infertility. Since spermatogenesis is such a complex process, it seems inevitable that many genes are involved in controlling the entire development of germ cells. Genes for infertility, however, are considered to be only those which are defected in the reproduction ability, but normal in other functions. Microdeletions of the Y chromosome have been observed frequently in infertile males. At least two genes, RBM and DAZ, are known to present in the loci where microdeletions occur frequently. A number of autosomal genes were also considered as candidates of infertility genes, based on phenotypes of knockout mice that were deficient of these genes.