Leveraging our previous study which subtyped ten BC cell lines [11], we performed quantitative real-time PCR (q-RT-PCR) to determine HDAC expression in models of luminal (RT4, SW780, and UMUC1) and basal–squamous (SCaBER, 5637, HT1376, and HT1197), as well as a subset of lines which did not fit into either luminal or basal–squamous subtyping schema (UMUC3, T24, and TCCSUP; hereafter referred to as “non-type”). This evidence concerns the gene HDAC9 and breast cancer.