Efforts to combine anti-CD20 antibodies (such as rituximab or obinutuzumab) with BCR inhibitors or venetoclax lead to the necessity to better understand the largely unclear mechanisms of CD20 regulation and its function (s) (reviewed in Pavlasova and Mraz1). This is underscored by the observation that in chronic lymphocytic leukemia (CLL) the combination of ibrutinib with rituximab fails to provide a clinical benefit in comparison to ibrutinib alone2 likely as ibrutinib downmodulates CD20 levels.1,3-5 PI3Kδ inhibitor idelalisib has been approved in combination with rituximab or ofatumumab;6 however, it remains unclear if idelalisib affects CD20 levels or function (s). Here we show for the first time that single-agent idelalisib therapy in CLL leads to CD20 downmodulation in vivo by interfering with a previously unknown mechanism of CD20 transcriptional regulation via the IL4- STAT6 axis. We describe a novel mechanism of CD20 regulation in CLL B cells, which has implications for combinatorial therapy with PI3K inhibitors.