Analysis of the phosphotyrosyl signal proteins before and after stimulation with Epo in clones with differing sensitivities to Epo. Cells were washed and either not stimulated or stimulated with Epo for 10 minutes. Cell lysates were immunoprecipitated with antiphosphotyrosine antibody 4G10 (PY). The immune complex was resolved by PAGE and transferred onto a nitrocellulose membrane. The blot was successively probed with antibodies against phosphotyrosine, STAT5b, and JAK2.