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lt; 0.05). Importantly, CLL cells obtained during ibrutinib therapy invivo were highly sensitive (90% apoptosis) to Akt inhibitor MK2206. RNA profiling of paired CLL samples obtained before and during ibrutinib (N = 22) or singleagent idelalisib therapy (N = 18) identified 16 differentially expressed mRNAs (with both drugs) involved in the PI3K-Akt pathway. Rictor induction was particularly notable since it is an essential assembly protein for mTORC2, which is known to phosphorylate Akt directly on S473 (Sarbassov et al. 2005). Analysis of samples obtained during therapy and genome-editing experiments in MEC1 cells revealed that transcription factor FoxO1 is directly responsible for Rictor/pAkt activation during ibrutinib treatment, and FOXO1 is required for adaptation to BTK inhibitors. FoxO1 inhibitor (AS1842856) decreased pAkt levels, induced apoptosis alone (~40% CLL cell killing) or more potently in combination with ibrutinib (~60% apoptosis; N = 7). In summary we describe the first non-genetic adaptation to targeted therapy with BCR inhibitors in CLL.