Component of the outer dense fibers (ODF) of spermatozoa. ODF are filamentous structures located on the outside of the axoneme in the midpiece and principal piece of the mammalian sperm tail and may help to maintain the passive elastic structures and elastic recoil of the sperm tail. Interacts (via leucine zipper motif) with TCP11 isoform 2 and isoform 3 (PubMed:21597245). Interacts with SPAG4 (By similarity). Interacts with KLC3 (By similarity). Testis. The C-terminal contains many C-X-P repeats.