BCAR3 (Breast Cancer Anti-estrogen Resistance 3) is an adaptor protein that functions as a key regulator of cell signaling pathways involved in proliferation, migration, and therapeutic resistance. BCAR3 acts within a co-regulatory circuit with Cas (p130Cas/BCAR1), where BCAR3 localization to integrin adhesions requires Cas, while Cas phosphorylation and downstream signaling require BCAR3 1. This interaction controls lamellipodia dynamics and cell migration through regulation of cytoskeletal reorganization 1. In cancer contexts, BCAR3 demonstrates dual roles depending on tumor type. In colorectal cancer, BCAR3 promotes cell proliferation, migration, and invasion 2. However, in multiple myeloma, high BCAR3 expression correlates with better patient prognosis and longer event-free survival 3. BCAR3 also mediates therapeutic resistance through multiple mechanisms: it confers cisplatin resistance in head and neck squamous cell carcinoma by sustaining TGF-β/SMAD signaling and suppressing mitochondria-derived apoptosis 4, and its expression correlates with hormonal therapy response in luminal breast cancer 5. In pulmonary fibrosis, BCAR3 orchestrates macrophage-fibroblast crosstalk by enhancing IL-4/Stat6 and TGF-β/Smad3 pathways, representing a potential therapeutic target 6. The protein's expression patterns vary across tissues, with differential roles in vascular versus tumor compartments 7.