PKD1 and breast carcinoma: Noteworthy, proliferation in estrogen-free medium of PKD1-overexpressing cells was not affected by control siRNAs but significantly reduced when transfected with PKD1-targeting siRNAs (Fig. 1E) clearly demonstrating that PKD1 overexpression allows estrogen-dependent MCF-7 breast cancer cells to proliferate in estrogen-free medium.