CAPG (capping actin protein, gelsolin-like) is a calcium-sensitive actin-binding protein that reversibly blocks the barbed ends of actin filaments 1. The protein exhibits calcium-dependent capping activity and is post-translationally modified through phosphorylation and acetylation 1. Beyond its canonical actin-regulatory function, CAPG is translated from circular RNA (circCAPG) into a 171-amino acid polypeptide that promotes triple-negative breast cancer growth by disrupting STK38-SMURF1 binding and activating the MEKK2-MEK1/2-ERK1/2 pathway 2. In pancreatic ductal adenocarcinoma, CAPG inhibition suppresses ferroptosis and promotes macrophage M1-like polarization, offering dual anti-tumor potential 3. Clinically, elevated CAPG expression strongly predicts poor prognosis across multiple cancer types, correlating with shorter overall survival, lymph node metastasis, and advanced TNM staging 4. In breast cancer, CAPG serves as a composite biomarker with GIPC1 to predict bone metastasis development and treatment response to zoledronic acid 5. CAPG promotes glioma proliferation, migration, and invasion 6 and is identified as a cerebrospinal fluid biomarker for microglial activation in neurodegenerative diseases including frontotemporal dementia and Alzheimer's disease 7. The protein shows promise as a cancer screening biomarker 8.