PGP and breast cancer: Therefore, we examined the phosphorylation status of CD44 in P-gp-positive and P-gp-negative cell lines using SKA, an ovarian carcinoma cell line that expresses CD44 but not P-gp and MCF-7/Adr breast cancer cells that express both CD44s and P-gp, and a specific antibody to the CD44 Ser325 phophorylated residue.