A gene expressed specifically in the testis has been unexpectedly isolated in the course of studies of the expression of a gene encoding an unknown protein that triggers cell death. The isolated gene was a novel gene sequence that had no significant homologue in the database. This gene was also found to be involved in the regulation of differentiation in the testis and to encode a protein, Tesmin, which may regulate the differentiation of spermatogeneous cells into primary spermatocytes. Tesmin proteins are described herein, as are antibodies that bind to such proteins.