Interleukin-11 (IL-11) is a profibrotic cytokine essential for the differentiation of fibroblasts into collagen-secreting, actin alpha 2, smooth muscle–positive (ACTA2+) myofibroblasts, driving processes underlying the pathogenesis of idiopathic pulmonary fibrosis (IPF). The gene discussed is ACTA2; the disease is idiopathic pulmonary fibrosis.