To understand the molecular mechanisms for the unresponsiveness of CD20 induction to HDAC inhibition by valproate, we investigated the levels of the activating histone mark H3K9ac and the repressive histone mark H3K27me3 on the CD20 promoter in circulating lymphoma cells from patients and in the matched del13q/NOTCH1wt CLL cell line I83-E95. The gene discussed is HDAC9; the disease is B-cell chronic lymphocytic leukemia.