Localization of GFP⁺ and mutant-BTK–expressing MEC-1 cells in mice. Tumors were established from MEC-1 cells harboring WT or mutant BTK, as described in Figure 4. (A-B) Representative flow cytometry plots for the spleen and bone marrow samples collected from mice with BTK^WT (n = 4; left), BTK^C481S (n = 9; center), and BTK^C481R (n = 9; right). At the time of euthanization, cells were collected from the spleen (A) and bone marrow (B), stained with a monoclonal antibody against human CD19 to identify the GFP⁺ MEC-1 cells, and analyzed by flow cytometry. (C-D) The percentage of CD19⁺ MEC-1 cells in spleen (C) and bone marrow (D). One-way analysis of variance, P = .05; Student t test, *P < .05 for BTK^WT vs BTK^C481R cells; **P < .01 for BTK^WT vs BTK^C481S cells.