Testis-specific promoters are unique in that relatively short proximal promoters of several genes have been shown to be capable of directing tissue- and cell-type-specific expression in transgenic mice. How such small promoter fragments perform the dual functions of maintaining a silenced state in somatic tissues and activating gene expression in the correct germ-cell type in testis remains poorly understood. Studies from our laboratory using the round spermatid-specific SP-10 gene as an experimental model have provided some insights into the mechanisms involved. It was found that the proximal promoter of the SP-10 gene acts as a chromatin insulator or boundary element in somatic tissues and prevents transcription of the SP-10 gene. In round spermatids, the insulator function is relieved, thus facilitating the SP-10 gene transcription. Insulators act as enhancer blockers and/or barriers to heterochromatin to protect the programmed expression of a gene. Typically, insulators are separable from promoters. In the case of the SP-10 gene, however, the insulator overlaps the promoter and operates in a facultative manner. We hypothesize that the proximal promoters of some testis-specific genes have adapted the insulator function to maintain transcriptional silence in the somatic tissues.