G-CSF–dependent Syk kinase phosphorylation occurs in WT G-CSF-R transfectants but not in transfectants expressing the truncated MT G-CSF-R. Murine 32D myeloid cells transfected with WT G-CSF-R or the truncated MT G-CSF-R, lacking the carboxy-terminal region of the receptor, were stimulated for 10 minutes with hG-CSF (300 ng/mL) and then lysed. SDS-PAGE was performed after immunoprecipitation of cell lysates with anti-Syk antibody. Blots were probed with antiphosphotyrosine antibody (A) and then stripped and reprobed with anti-Syk (B), anti-Lyn (C), or anti-Hck (D) antibodies. pSyk indicates phosphorylated Syk.