Azoospermia is the most frequent cause of male infertility. After excluding the obvious urological reasons and the effect of Klinefelter's syndrome, azoospermia may be caused by an abnormality in the crucial gene(s) expressed during male germ cell differentiation. Recently, two candidate genes for azoospermia have been cloned from the azoospermic factor (AZF) locus on the Y chromosome long arm (Yq). One is YRRM (Y chromosome RNA recognition motif) gene, and the other is DAZ (deletion in azoospermia) gene. Both genes encode RNA binding protein and their expression is restricted to the testis. Therefore they are good candidates for AZF, although their function remains unclear. Here, the genes on the Y chromosome possibly involved in spermatogenesis and the role of the Y chromosome in evolution are discussed.