To elucidate the molecular mechanism of these clinical observations, we tested, using a metabolic survival assay, the effect of MDM2-gene-silencing (siMDM2#1, siMDM2#2) on a panel of breast cancer cell lines: MCF7 (luminal subtype, with wild-type TP53), SKBR3 (HER2-enriched subtype with mutated p53-R175H), MDA-MB-231 (basal subtype, with mutated p53-R280K), and MDA-MB-468 (basal subtype with mutated p53-R273H). The gene discussed is ERBB2; the disease is breast cancer.