What permits some cancer cells in a tumor to resist treatment while other cancer cells die from the treatment?

Cancer is a diseases that remains one of the main causes of death in human and  different methods have being developed to treat cancer.  Thus, drug resistance leads to the failure of tumor treatment while the expression of ATP-binding casette (ABC) is related to the outcome of chemotherapy in cancer treatment. However, the higher the expression of ABC in a tumor the more resistant cancer cells to chemotherapy.