Two potential mechanisms for the development of drug resistance. Inherent drug resistance refers to the presence of a small number of resistant cells in the original tumor cells. After receiving chemotherapy or targeted therapy, due to the high adaptability of this small number of cells, they gradually proliferate and develop into the main dominant group, which is the drug-resistant group. Acquired drug resistance refers to the absence of resistant cells in the original tumor cells. The pressure of chemotherapy or targeted therapy stimulates a small number of cells to acquire drug resistance through gene mutation, epigenetic modification, and other ways, and eventually proliferate and develop into the main dominant group, producing drug resistance.