The deduced 345-amino acid human protein contains a fibronectin type III domain in the N terminus and 5 ankyrin repeats in the C terminus. The protein is highly conserved in vertebrate species. Mammalian orthologs share 85 to 99% sequence identity. Northern blot analysis in mouse and human tissues demonstrated testis-specific expression. Studies of mouse embryos suggested that Fank1 expression in the developing testis occurred in pachytene spermatocytes and round spermatids. Antibodies raised against mouse Fank1 detected the expected approximately 38-kD band in nuclei of pachytene spermatocytes and spermatids. FANK1 expression coincided with the transition from the meiotic phase of spermatogenesis to the haploid phase, suggesting that FANK1 may have a role in late-meiotic or postmeiotic processes.