ACTA1 and hamartoma: Pulmonary SMH is a rare benign lesion characterized by the hyperplastic proliferation of smooth muscle bundles forming small nodules.1,2) Histologically, SMH is composed of spindle-shaped smooth muscle cells arranged in fascicles, and it typically expresses markers such as α-SMA, desmin, and h-caldesmon.1–4) Other benign pulmonary lesions with spindle cell morphology include hamartoma, leiomyoma, and inflammatory myofibroblastic tumor.