FNDC3A is a member of a three-gene family in mice. It is expressed in several tissues including testis, and encodes a novel protein composed of a proline-rich amino-terminus, nine fibronectin type-III domains, and a hydrophobic carboxy-terminus. The proline-rich region of each family member contains conserved amino acids that include a PPGY consensus binding site for type I WW domain containing proteins. The hydrophobic carboxy-terminus is similar to that found in 'tail-anchored' proteins, integral membrane proteins that are localized to the cytosolic face of the endoplasmic reticulum. Immunohistochemical staining has shown that FNDC3A localizes to the acrosome of spermatids, as well as to Leydig cells in the mouse testis. FNDC3A may be involved in mediating spermatid-Sertoli adhesion during mouse spermatogenesis and mutation of FNDC3A is the cause of male sterility in sys mice. FNDC3A has also been found to be overexpressed in human odontoblasts.