FSIP1 (fibrous sheath interacting protein 1) is an oncogene frequently overexpressed across multiple cancer types, functioning as a critical regulator of tumor progression and therapeutic resistance. Originally identified as a spermatogenesis-related testicular antigen, FSIP1 is aberrantly expressed in breast, colon, gastric, and non-small cell lung cancers 1234. Mechanistically, FSIP1 promotes cancer progression through multiple pathways: it binds directly to HER2's intracellular domain to enhance proliferation and invasiveness 5, stabilizes the multidrug resistance protein 1 (MRP1) to confer chemotherapy resistance 6, and regulates autophagy via ULK1 interaction to modulate drug sensitivity 7. Clinically, high FSIP1 expression independently predicts poor prognosis across multiple malignancies, correlating with advanced pathological stage, lymph node metastasis, and reduced overall survival 123. In breast cancer specifically, FSIP1 associates with HER2-positivity and docetaxel resistance, making it a potential therapeutic target 6. FSIP1 expression can improve prognostic prediction beyond conventional staging systems 4, suggesting utility as a biomarker for patient stratification and treatment planning.