BTK^Cys481Sermutation promotes ibrutinib resistance and persistent activation of BTK and ERK1/2 signaling in MYD88-mutated WM and ABC DLBCL cells. MYD88-mutated WM (BCWM.1, MWCL-1) and ABC DLBCL (TMD8, HBL1) cells were transduced with vector alone, or vectors expressing either BTK^WT or BTK^Cys481Ser (BTK^C481S). Dose-dependent survival determined by CellTiter-Glo Luminescent cell viability assay for vector only, BTK^WT, or BTK^C481S transduced MYD88-mutated WM and ABC DLBCL cells following treatment with ibrutinib for 72 hours. Each data point was calculated relative to DMSO control. The x-axis shows ibrutinib log concentration (A). Similar levels of total BTK protein expression were obtained in MYD88-mutated WM and ABC DLBCL cells following transduction with either BTK^WT or BTK^C481S vectors (B). Cell viabilities at 72 hours for vector only, BTK^WT, or BTK^Cys481Ser transduced MYD88-mutated WM and ABC DLBCL cells following treatment with ibrutinib for 6 hours and then washout and replacement of media without ibrutinib. Each data point was calculated relative to DMSO control. The x-axis shows ibrutinib log concentration (C). Immunoblotting studies depicting impact of vector only, BTK^WT, or BTK^C481S expression in MYD88-mutated WM and ABC DLBCL cells on BTK, PLCγ2, ERK1/2, and p90RSK signaling following treatment with vehicle control (DMSO) or ibrutinib (0.5, 0.1 μM) for 2 hours. GAPDH used as protein loading control (D). RLU, relative luminescence unit.