CTSK and mesenchymal cell neoplasm: However, while carcinomas (without TFE3 fusions) are usually cathepsin K negative, mesenchymal neoplasms can frequently show cathepsin K staining (e.g., more than half of 414 non-ASPS mesenchymal tumors in one series), including many cases of melanoma (which is also within the morphologic differential diagnosis for ASPS) [14].