Cathepsin K is an established immunohistochemical marker in the classification of primary renal neoplasms, as demonstrated by the International Society of Urological Pathology (ISUP) recommendations, to identify translocation renal cell carcinoma and the spectrum of perivascular epithelioid lesions, in particular pure epithelioid PEComa/epithelioid angiomyolipoma [1]. The gene discussed is CTSK; the disease is renal cell carcinoma.