EMT6/AR1 was derived from the mouse mammary tumour cell line EMT6/P by growth in increasing amounts of doxorubicin over a period of 8 weeks. The cell line showed a 40-fold greater resistance to doxorubicin, which is also known as adriamycin. The line was reported to express a classical multi-drug resistance (MDR) phenotype with hyperexpression of membrane P-glycoprotein and a spectrum of cross-resistance e.g. against vincristine and colchicine. Overexpression of CP22 (sorcin) and mdr 1a gene was reported. Cells were shown to have a reduced intracellular drug accumulation and exhibit a doubling time of 12-15 hours. It is recommended to culture the cells without drug after resuscitation until the first passage.