Spermiogenesis is the terminal differentiation process of the male germ cell during which haploid spermatids acquire unique structures such as the acrosome and flagellum and undergo extensive cellular reorganization. Although well described morphologically, the molecular mechanisms underlying spermiogenesis are not well understood. The SP-10 gene, which codes for the acrosomal protein SP-10, has been well characterized in mice and men. This single copy gene is localized to syntenic regions of chromosomes 9 and 11 in mouse and human, respectively. The SP-10 gene is testis-specific, and is transcribed and translated in round spermatids. The differentiation marker SP-10 serves as a useful model to address questions regarding the regulation of round spermatid-specific gene transcription and acrosome biogenesis. This paper defines the temporal pattern of SP-10 gene expression during spermiogenesis and reviews the work done on analysis of the SP-10 promoter. Transgenic mice demonstrated that either the -408/+28 or the -266/+28 region of the SP-10 promoter could drive round spermatid-specific expression of a GFP reporter gene whereas the -91/+28 region lacked promoter activity. The transgene expression mimicked the spatial and temporal patterns of expression of the endogenous SP-10 gene. Surprisingly, none of the transgenic lines showed expression of GFP in tissues other than testis. Given the complexity of eukaryotic transcriptional regulation, the fact that a short 294-bp promoter is capable of conferring developmental stage- and cell type-specific transcription of a gene is intriguing and paradoxical.