In regard to IHC, moderate (++) to strong (+++) TFE3 nuclear positivity is the primary clue to the initial diagnosis of Xp11.2 tRCC and cathepsin K positivity seems to be relevant to PRCC-TFE3 subtype. The gene discussed is CTSK; the disease is renal cell carcinoma associated with Xp11.2 translocations/TFE3 gene fusions.