The different expression of cathepsin K between alveolar soft part sarcoma and ASPSCR1-TFE3 rearranged renal cell carcinoma has been explained by a subtle difference in the chromosome translocation, unbalanced in alveolar soft part sarcoma, and consistently balanced in ASPSCR1-TFE3 rearranged renal cell carcinoma. The gene discussed is CTSK; the disease is renal cell carcinoma.