Microtubule-binding protein that localizes to the microtubular manchette of elongating spermatids. Homodimer (PubMed:28283191). Interacts with HOOK1 (PubMed:28283191). Interacts with HOOK2 (PubMed:28283191). Interacts with HOOK3 (PubMed:28283191). Localizes to the microtubular manchette of elongating spermatids (PubMed:28283191). Localizes to the sperm flagella and to the basal half of motile cilia (PubMed:28283191). Predominantly expressed in testis (PubMed:28283191). Expressed at lower level in brain, eye, trachea and lung (PubMed:28283191). Barely expressed in tongue, heart, liver, kidney, spleen and muscle (PubMed:28283191). Present at high level in elongating spermatids, whereas lower levels are observed in round spermatids (at protein level) (PubMed:28283191). In testis, expressed at low level until day 20, when round spermatids appear for the first time. After day 20, a sharp and constant increase of expression is observed. Belongs to the CCDC181 family.