CSAG2 (CSAG family member 2) is a cancer-testis antigen located on chromosome X that functions as a drug-resistance-related protein associated with chemotherapy resistance and malignant phenotypes 1. CSAG2 was identified as one of the top 10 metastasis-related genes in gastric cancer and is among genes predictive of paclitaxel sensitivity in breast cancer 23. Mechanistically, CSAG2 belongs to the CSAG gene family, which shares a common ancestral origin with VCX/Y and SPANX families, featuring conserved promoter elements and predominant expression in spermatids with weak somatic expression 4. Clinically, elevated CSAG2 expression is highly predictive of poor prognosis: in ovarian cancer patients receiving paclitaxel-based chemotherapy, weak CSAG2 expression correlated with superior overall and progression-free survival (P<0.001 and P=0.003, respectively) 1. CSAG2 expression is significantly elevated in aggressive prostate cancer (Gleason score 8-10) 5, osteosarcoma with poor survival outcomes 6, cervical cancer 7, and melanoma 8. These findings establish CSAG2 as a prognostic biomarker and potential immunotherapy target across multiple malignancies, with expression patterns indicating tumor-associated rather than normal tissue expression.