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 (By similarity). Interacts with SPAG4 (PubMed:10373309). Interacts with KLC3 (PubMed:12594206). Testis. Specifically located to the round spermatid layer and to the luminally-oriented cytoplasm of elongated spermatids. First detected in 30-day old rats after which, levels increase during spermatid elongation. Levels decrease at the time of spermatid assembly and disappear just before spermiation. The C-terminal contains many C-X-P repeats.