TCF21 is a basic helix-loop-helix (bHLH) transcription factor that plays critical roles in fibroblast biology and cardiovascular disease protection 1. In vascular smooth muscle cells, TCF21 promotes atheroprotective phenotypic modulation, facilitating transformation into fibromyocyte-like cells that contribute to protective fibrous cap formation in atherosclerotic lesions 2. Higher TCF21 expression is associated with decreased coronary artery disease risk in humans 2. In cardiac tissue, TCF21 regulates fibroblast differentiation and maintains fibroblast identity during development 1. Enforced TCF21 expression in cardiac fibroblasts inhibits myofibroblast differentiation and significantly reduces cardiac fibrosis and hypertrophy following injury 3. In pulmonary fibrosis, TCF21 functions as a lipogenic transcription factor that maintains the homeostatic lipofibroblast phenotype, and its downregulation by NNMT promotes transition to pro-fibrotic myofibroblasts 4. The gene is also implicated in atrial fibrillation susceptibility, with certain TCF21 polymorphisms and elevated serum levels associated with increased disease risk 5. Additionally, TCF21 acts as a tumor suppressor in gastric cancer, inhibiting proliferation and enhancing chemosensitivity through AKT pathway modulation 6.