The breast cancer cell lines MDA-MB-231 (HER-2−/ER−/PR−), MCF7 (HER-2−/ER+/PR+) and SKBR3 (HER-2+/ER−/PR−), representing the main molecular subtypes of breast cancers as portraited earlier, were employed to generate the CLDN9 knockdown cell models. Here, CLDN9 is linked to breast cancer.