Component of the manchette, a microtubule-based structure which plays a key role in sperm head morphogenesis during late stages of sperm development (PubMed:19091768, PubMed:28003339). Important for male fertility (PubMed:19091768). Interacts with FASLG (By similarity). Interacts with LRGUK (via guanylate kinase-like domain) (PubMed:28003339). Interacts (via C-terminus) with HOOK1 (via coiled-coil region) (PubMed:19091768). In elongating spermatids, localizes to the manchette. Specifically expressed in testis, where it localizes to postmeiotic germ cells (at protein level). Detected in testis from postnatal day 20 onwards (at protein level). In developing spermatozoa, weakly expressed in pachytene spermatocytes and round spermatids, and highly expressed in elongating spermatids (at protein level). Detected in residual bodies but not mature sperm (at protein level). Males are almost completely infertile, but otherwise have no visible phenotype. Sperm morphology is highly abnormal with deformed nuclei, detached acrosomes and an expanded perinuclear space. Defects are first apparent from the round spermatid stage and are associated with abnormal development of the manchette. Belongs to the RIMBP family. Truncated N-terminus. Truncated N-terminus.