CD150-initiated ERK1/2 phosphorylation was not affected by SH2D1A cotransfection but was dependent on SHIP. (A) Lyn^-/- CD150⁺, Btk^-/- CD150⁺, and SHP-2^-/- CD150⁺ cells had elevated basal levels of ERK phosphorylation, and ERK phosphorylation did not exceed control levels after CD150 ligation. (B) WT DT40 cells and DT40 cells transfected with empty pApuro or pApuro/pcDNA3 vectors served as negative controls. Each sample contained lysate from 10⁶ cells. Equal loading was monitored in all experiments by anti-p38 MAPK kinase or anti-actin Western blot (control panels). One of 5 experiments.