ABCB1 and osteosarcoma: One of the most common phenotypic traits associated with the drug-resistant phenotype in osteosarcoma is the overexpression of efflux pumps of the ATP-binding cassette (ABC) family, such as ABCG2 (also known as BCRP) and ABCB1 (also known as MDR1 or P-glycoprotein), the expression of which is associated with a lower event-free survival rate in osteosarcoma patients [14,15,16].