CTSK and adenocarcinoma: Although negative in the neoplastic cell, staining for cathepsin K is not uncommonly seen in the peritumoral stroma of adenocarcinomas arising in different organs (esophagus, stomach, colon, pancreas, biliary tract, lung, breast, and ovary), especially in the elongated spindle cells present in the desmoplastic stroma, morphologically considered as reactive myofibroblasts.