Spermatogenesis is an orderly process whereby male germ cells pass through sequential phases of differentiation to develop into mature spermatozoa. At present, little is known about the factors that regulate this process. The POU domain represents a group of transcription-activator proteins with well-described roles in development. These proteins have been shown to regulate the tissue-specific expression of genes involved in cellular differentiation. The recent identification of Sperm-1, a testis-specific POU-domain protein expressed in primary spermatocytes just prior to the onset of meiosis, suggests that this protein may be involved in the regulation of male germ-cell differentiation. Continued evaluation of Sperm-1 and other related transcriptional activators may shed some light on the regulation of spermatogenesis.