In the course of our interrogation of the mechanism by which HDAC inhibitors suppress CSC-like properties in breast cancer cells, we obtained evidence that HDAC8, a class I HDAC, plays a pivotal role in maintaining Notch1 protein stability by protecting against Fbw7-mediated proteasomal degradation, independently of its deacetylation activity. This evidence concerns the gene HDAC9 and breast cancer.