ACTA1 and neoplasm: Myofibroblastic CAFs (myCAFs) are α-SMA-high, TGF-β-responsive stromal cells that reside close to tumor nests and are major producers of the extracellular matrix (e.g., collagen), generating a dense, stiff stroma that impedes drug delivery and CD8+ T cell trafficking; accordingly, tumors enriched for myCAFs often show an abundant ECM and worse prognosis.