FSCN3 (fascin actin-bundling protein 3) is a testis-specific actin-bundling protein localized to chromosome 7.3 that plays crucial roles in spermatogenesis and male fertility. The protein shares structural homology with fascin family members, encoding a 498 amino acid protein of approximately 56 kDa 1. FSCN3 is specifically expressed in elongating spermatids, where it localizes to the anterior spermatid head in subacrosomal spaces and functions in microfilament reorganization during spermiogenesis 1. The protein is retained in mature spermatozoa, suggesting involvement in fertilization-related processes 1. In terms of disease relevance, FSCN3 is significantly downregulated in non-obstructive azoospermia (NOA), a condition characterized by impaired spermatogenesis 2. It has been identified as a hub gene associated with spermatogenesis, with expression levels and methylation patterns linked to testicular pathology and infertility 2 3. Beyond reproductive function, recent exome-wide association studies identified FSCN3 as a novel locus associated with back pain phenotypes through loss-of-function variants, representing a second gene mechanism for this condition and suggesting broader musculoskeletal relevance 4. These findings indicate FSCN3 as a potential therapeutic target for both male infertility and back pain-related disorders.