B2M and breast cancer: (4) β2-M siRNAs have different silencing effects in the different breast cancer molecular subtypes; it significantly inhibited Bcl-2 mRNA expression and did not inhibit the ER, PR and HER-2 mRNA expression in MCF-7 cells (ER+, PR+ and HER-2−); however, there was significant up-regulation in the Bcl-2 and HER-2 mRNA expression levels in MDA-MB-231 cells (ER−, PR− and HER-2−), which is also consistent with the silencing effect at the protein level.