To study whether the CD44-mediated protection of P-gp from ubiquitination is also present in mammalian cells, we tested the stability of P-gp using a cycloheximide chase analysis in a breast cancer cell line that expresses P-gp but not CD44 (MCF-7/BC19) and in a cell line that expresses both P-gp and CD44 (MCF-7/Adr). This evidence concerns the gene CD44 and breast cancer.