The 17q-linked breast and ovarian cancer susceptibility gene (BRCA1) is believed to function as a tumor suppressor gene (Miki et al., 1994). In this report BRCA1 RNA expression has been analysed in adult mouse tissues with detailed attention to its expression in prepuberal and adult testis. Measurements of BRCA1 mRNA levels in highly purified somatic cells of the testis and in staged germ cells showed that high level BRCA1 mRNA expression is limited to the germ cells. Within the germ cell lineage, the high level expression was detected in meiotic cells, specifically pachytene spermatocytes and in post-meiotic round spermatids. This is in contrast to premeiotic germ cells which were found to express little or no BRCA1 mRNA. These observations, considered together with recent data on the expression of BRCA1 in breast epithelium, argues against a function for BRACA1 in early progenitor cells in both tissues and cells attention instead to roles intimately associated with terminal differentiation or with final rounds of cell division.