In chronic lymphocytic leukemia (CLL), NOTCH1 mutations have been associated with clinical resistance to the anti-CD20 rituximab, although the mechanisms behind this peculiar behavior remain to be clarified. In a wide CLL series (n=692), we demonstrated that CLL cells from NOTCH1 mutated cases (87/692) were characterized by lower CD20 expression, and lower relative lysis induced by anti-CD20 exposure in-vitro. Consistently, CD20 expression by CLL cells was up-regulated in-vitro by γ-secretase inhibitors or NOTCH1-specific siRNA, and the stable transfection of a mutated (c.7541-7542delCT) NOTCH1 intracellular domain (NICD-mut) into CLL-like cells resulted in a strong downregulation of both CD20 protein and transcript. By using these NICD-mut transfectants, we investigated protein interactions of RBPJ, a transcription factor acting either as activator or repressor of NOTCH1 pathway when respectively bound to NICD or hystone deacetylases (HDACs). Compared to controls, NICD-mut transfectants had RBPJ preferentially complexed to NICD, and showed higher levels of HDACs interacting with the promoter of the CD20 gene. Finally, treatment with the HDAC inhibitor valproic acid upregulated CD20 in both NICD-mut transfectants and primary CLL cells. In conclusion, NOTCH1 mutations are associated with low CD20 levels in CLL and are responsible for a dysregulation of HDAC-mediated epigenetic repression of CD20 expression.Leukemia accepted article preview online, 13 July 2015. doi:10.1038/leu.2015.182.