For the unique, highly DOXO-resistant line HOS-R/DOXO line (RI = 224), we observed a multi-drug resistant phenotype, with cross-resistance to other agents used in osteosarcoma treatment and substrates of PgP (e.g., ETOP, VCR, but also to MTX), associated with ABCB1/MDR1 gain translated in high MDR1 mRNA and PgP protein expression. This evidence concerns the gene ABCB1 and osteosarcoma.