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Context: Ibrutinib inhibits BTK with the most typical response in CLL being partial remission (PR) with measurable minimal residual disease (MRD) in blood. Remission is usually maintained until development of genetically-driven resistance. Objective: Mechanism of adaptation and survival in MRD. Patients and Methods: Pre-treatment CLL cells and under-treatment MRD were systematically collected from 33 high-risk CLL patients. Samples were characterized by high-dimensional, single-cell flow cytometry; bulk genomic, transcriptomic, and chromatin accessibility profiling; and single-cell RNA-seq profiling. Results: The genetic composition of MRD remained constant across timepoints, indicating that ibrutinib did not have any impact in shaping its genomic diversity. Ibrutinib induced chromatin dynamics in MRD resulting in a predominantly closed state. Chromatin regions that became more accessible under ibrutinib were enriched of binding sites for transcription factor that lay downstream ERK signaling. Regions that turned into a less accessible state were enriched for the binding sites of TF emanating from the BCR. At the transcriptomic level, most of the differentially expressed genes between pre- and post-treatment samples were downregulated in MRD, while only a fraction was upregulated. IL4, NF-kB, and metabolism and proliferation signatures were downregulated in MRD, while MAPK and RAS signatures were upregulated. CLL spleen samples from TCL1 mice under ibrutinib treatment showed an upregulation of MAPK pathway genes compared to untreated mice. Single-cell transcriptomes revealed a distinct post-treatment cell population driven by MAPK activity that functionally pre-existed in a fraction of CLL cells of baseline samples before ibrutinib start. At the signaling level, ibrutinib had no effect on the integrity of RAS-BRAF-MAPK-ERK pathway protein expression upon crosslinking of the BCR with anti-IgM but not after stimulation of TLR9 or CD40. Functional validation on primary samples by western blotting shows that RAS/ERK can be activated by BCR stimulation, irrespective of ibrutinib treatment in ex vivo experiments. Pharmacological inhibition of MEK (trametinib) and ERK (ulixertinib) synergized with ibrutinib, leading to decreased cell viability. Conclusions: MRD under ibrutinib adapts its phenotype in an epigenetic way to maintain functional competence of BCR signaling via the MAPK pathway, which turned to be a vulnerability of MRD persisting under ibrutinib.