Colony-stimulating factor 1 receptor (CSF1R) is tyrosine phosphorylated in MKPL-1 cells. (A) Immunoblot analysis of whole-cell lysates revealed constitutive tyrosine phosphorylation of STAT5 in the MKPL-1 cell line. Detection of phospho-STAT5 is shown in the upper panel and total STAT5 is shown below. (B) Detection of a truncated form of CSF1R in MKPL-1 cells by Western blot. ▶ indicates full-length CSF1R; *, a nonspecific band; and →, a truncated form of CSF1R. (C) Treatment of MKPL-1 cells with increasing concentrations of imatinib resulted in a decrease in proliferation. (D) Treatment with imatinib increased apoptosis of MKPL-1 cells but not CMK cells as measured by cleaved caspase-3 staining. (E) Incubation with imatinib resulted in decreased phosphorylation of a truncated form of CSF1R in MKPL-1 cells, accompanied by decreased phosphorylation of STAT5 and ERK.