Mammalian spermatogenesis is a complex developmental process. Spermatozoa, like ova, are uniquely capable of supporting embryonic development. Our approach to understanding this process is to identify genes whose developmental pattern of expression suggests that they may play a role in spermatogenesis. Experiments on the cellular oncogene c-abl and the homeo box-containing gene Hox-1.4 indicate that these genes may be important for male germ cell development. Both genes produce testis-specific transcripts that are present in particular cellular populations of the adult testis. Their developmental specificity, however, is different: c-abl is haploid-specific, whereas Hox-1.4 is expressed in the germ cells as soon as they have entered meiosis. Future studies will focus on examining the protein products of these genes and their function in testicular cells.