Compared with smooth muscle tumors, PEComas have key features of eosinophilic to clear cytoplasm, round to oval nuclei, a prominent capillary network, and strong immunoreactivity for melan-A, HMB45, melanocyte-inducing transcription factor, and cathepsin K. Furthermore, PEComas commonly stain negatively for epithelial membrane antigen and cytokeratin, unlike epithelioid smooth muscle tumors. The gene discussed is CTSK; the disease is neoplasm with perivascular epithelioid cell differentiation.