ACTL7B (actin-like 7B) is a testis-specific actin-related protein (Arp) encoded by an intronless gene on chromosome 9 1. Unlike its paralog ACTL7A, which is widely expressed, ACTL7B is specifically expressed in the testis and prostate, with highest levels in round and elongating spermatids during spermiogenesis 1. The protein localizes to the developing acrosome, spermatid nucleus, and flagellum connecting region 2. ACTL7B functions as a critical regulator of spermatid morphogenesis through interaction with LC8 dynein light chains (DYNLL1 and DYNLL2), which are essential for proper intracellular transport and cytoskeletal organization 3. ACTL7B-deficient mice are infertile with severe spermatogenic defects including acrosomal detachment, membrane disruption, and flagellar malformations beginning at spermiogenesis step 9, ultimately resulting in spermatogenic arrest and autophagy-mediated degradation of abnormal spermatids 32. In humans, ACTL7B loss results in oligoteratozoospermia (OAT) and multiple morphological abnormalities of the flagellum (MMAF), phenotypes that mirror idiopathic male infertility 2. ACTL7B levels are dysregulated in non-obstructive azoospermia patients and may serve as a diagnostic biomarker 4. These findings establish ACTL7B mutations as direct causes of male infertility, with clinical relevance for infertility diagnosis and treatment.