PGP and osteosarcoma: Nowadays, many cell lines and primary tumors have been tested for both P-gp expression and resistance to these agents, whereby a positive correlation has been established (i.e., high P-gp expression is associated with more chemotherapy resistance) across many different cancer types such as breast and ovarian cancers [84], multiple myeloma [85], osteosarcoma [86], and lung cancer [87] to name a few.