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. The gene discussed is HDAC9; the disease is breast carcinoma.