To evaluate the status of the inactive X chromosome in different types of breast cancer, we selected three cell lines that represent the main breast cancer molecular subtypes: ZR-75-1 (luminal), SK-BR-3 (HER2+), and MDA-MB-436 (Basal-Like Carcinoma [BLC], BRCA1 null). This evidence concerns the gene ERBB2 and breast carcinoma.