Fank1 is a testis-specific gene encoding a nuclear protein exclusively expressed during the transition from the meiotic to the haploid phase of spermatogenesis.

In our effort to identify testis-specific genes we found Fank1, which encodes a protein containing a fibronectin type III domain in the amino terminus and five ankyrin repeats in its carboxyl terminus. FANK1 displays a high degree of sequence conservation in 11 vertebrate species during evolution. Bioinformatic and experimental analyses revealed that Fank1 was exclusively expressed in the testis in both mice and humans. Fank1 mRNA was expressed in mid to late pachytene spermatocytes as well as spermatids in steps 1-14. FANK1 protein was localized in the nuclei of the same cells within the seminiferous epithelium. Consistent with its nuclear localization, a gene ontology analysis suggests that FANK1 has a DNA binding activity and thus may function as a transcription factor. Given the highly restricted expression of FANK1, it may have a role in regulating gene expression in the transition from the meiotic phase to the haploid phase during spermatogenesis.