In some tumor types including hemangiopericytoma, apocrine gland adenocarcinoma, hepatoma, cholangiocarcinoma, transitional cell carcinoma, adrenal gland adenoma, colorectal adenoma, and adenocarcinoma, more than 50% of the canine cases were positive for P-gp expression as determined by immunohistochemistry. This evidence concerns the gene PGP and cholangiocarcinoma.