Using a positional cloning approach, we have isolated a new gene family, the Y-located RNA Recognition Motif genes (YRRM), which constitutes a candidate for AZF, the 'azoospermia factor' located principally within band Yq11.23, and thought to be important in the control of human spermatogenesis. The YRRM gene family has at least 15 members, more than one of which are transcribed, some of which are pseudogenes. RNA in situ hybridization to adult human testis tissue indicates that gene expression of the YRRM family is confined to germ cells, notably spermatogonia and/or primary spermatocytes. Similar patterns of hybridization are seen for a second gene family, TSPY, clustered mainly on the short arm of the human Y chromosome. Both YRRM and TSPY show Y chromosome conservation in several mammalian species.