TPO and GM-CSF induced the tyrosine phosphorylation of GAB1. UT-7 cells stably transfected with the TPO receptor (c-mpl) and parental UT-7 cells were stimulated for 10 minutes with 50 ng/mL TPO, 10 U/mL Epo, or 25 ng/mL GM-CSF. Cleared lysates were immunoprecipitated with anti-GAB1 antibodies and immunoprecipitates were analyzed by Western blot using antiphosphotyrosine antibodies (anti-PY).