Coimmunoprecipitation studies identifying phospho-BTK as a binding partner of MYD88 in L265P-expressing WM cells, and abrogation of MYD88-BTK binding following treatment with ibrutinib in MYD88 L265P-expressing cells. (A) Immunoprecipitation experiments using pull-down (IP) anti-BTK– or phospho-BTK–specific antibodies followed by IB with an anti-MYD88 antibody in lysates from MYD88 L265P heterozygous-expressing BCWM.1 and MWCL-1 cells, and MYD88 WT-expressing OCI-Ly19 and Ramos cells. The ratio of MYD88 vs total BTK was analyzed by densitometry showing the amount of MYD88 protein that was pulled down by BTK. (B) Depicts experiment in which IP was performed with anti-MYD88 antibody and IB with an anti-BTK antibody. (C) Impact of ibrutinib pretreatment (4 μM for 90 minutes) on coimmunoprecipitation of BTK with MYD88 in MYD88 L265P-expressing BCWM.1, MWCL-1, and OCI-Ly3 cells, and MYD88 WT-expressing OCY-Ly19, Ramos, and RPMI 8226 cells. IB, immunoblotting antibody; IP, immunoprecipitation antibody.