In order to eliminate factors other than P-gp that may develop during the long time necessary for a cell line to develop P-gp+ MDR during incremental drug challenge of parental drug sensitive cell lines; human and murine MDR1 cDNA were transfected into a metastatic breast carcinoma cell line and P-gp was allowed to demonstrate itself under the selecting pressure of 3 different drugs at four different concentrations in a short time compared to the previous MDR lines developed without MDR1 transfection. This evidence concerns the gene PGP and breast carcinoma.