We first compared the relative endogenous Notch3 expression levels by western blotting in a panel of breast cancer cell lines previously classified as four subtypes, namely MCF-7 and T47D (ERα/PgR-positive luminal mammary carcinoma cells), SKBR3 (Her2 (Neu/ErbB-2)-overexpressing human breast cancer cell line), MDA-MB-231 (G and Z indicating two strains stored in different laboratories) and BT549 (triple-negative/basal-like breast cancer cells, TNBC). The gene discussed is NOTCH3; the disease is breast cancer.