B2M and breast carcinoma: Third, β2M exhibits different regulatory effects on CREB and p-CREB in the cell lines of ER+ breast cancer with HER2− (MCF-7 and T47D), and HER2 and Bcl-2 in the cell lines of ER− breast cancer with HER2− (MDA-MB-231 and Hs578T), which may be due to the fact that breast cancer has a very complex molecular regulatory mechanism, and every type of breast cancer has several subtypes, and these signaling molecules may be regulated by other pathways in corresponding subtypes of breast cancer.