ROPN1B (rhophilin associated tail protein 1B) is a sperm-specific protein with critical roles in male fertility and emerging importance as a cancer-associated antigen. In reproductive biology, ROPN1B functions in fibrous sheath integrity and flagellar structure, contributing to sperm motility through PKA-dependent signaling during spermatozoa capacitation 1. The protein is consistently identified as a key marker in proteomic studies of male infertility, appearing among candidate targets for monitoring sperm quality across conditions including asthenozoospermia and poor sperm quality 1. ROPN1B expression is dysregulated in smokers following COVID-19 infection, correlating with impaired spermatozoal function 2, and shows altered expression in testosterone deficiency-related male hypogonadism 3, suggesting its testosterone-dependent regulation. Beyond reproductive function, ROPN1B exhibits unexpected expression in human malignancies. It is highly expressed in triple-negative breast cancer 4 and is part of characteristic SOX10 gene signatures in adenoid cystic carcinoma and basal-like breast carcinomas 5. Most notably, ROPN1B functions as an immunogenic cancer-restricted antigen in melanoma, with 88.5% of melanoma patients expressing ROPN1B and 71.2% developing antibody responses 6. This makes ROPN1B a viable target for cancer immunotherapy, potentially combined with immune checkpoint inhibitors 6. However, tumor-associated anti-ROPN1B antibodies show limited predictive value for immunotherapy response in melanoma 7.