EMT6/VCR/R is a drug-resistant variant of the mouse mammary tumour cell line EMT6/P. It was derived from the parent line by continuous growth in increasing concentrations of vincristine. EMT6/VCR/R was reported to be cross-resistant to other drugs of the multidrug-resistance (MDR) phenotype, showed reduced intracellular drug accumulation and hyperexpression of P-glycoprotein.