HDAC9 and breast cancer: As HDAC inhibitors are reported to release NO [21], the findings that 1,25D in combination with HDAC inhibitors resulted in re-expression of anti-proliferative target genes in TN breast cancer cell line MDA-MB-231 [40] support our findings that combinatorial therapy using 1,25D and low physiological dose of NO may reduce the proliferative ability of MCSCs and thereby inhibit tumor initiation and tumor mass.