We investigated the role of CLDN1 by examining two subtypes of breast cancer using a luminal A subtype of breast cancer cell line MCF7, which was positive for estrogen (ER) and progesterone receptors (PR), negative for Her2 and a basal-like subtype MDA-MB-231 cells, and negative for ER, PR and Her2. The gene discussed is ERBB2; the disease is breast cancer.