STAT5A/WKR and STAT5A/▵53C block G-CSF–induced neutrophil differentiation of 32D cells. (A) 32D cells expressing either the neomycin resistance gene alone (32D/Neo), STAT5A/WKR (32D/WKR), or STAT5A/▵53C (32D/▵53C) were placed in 100 ng/mL human recombinant G-CSF and minimal IL-3 to permit cell viability throughout the differentiation assay. Wright-Giemsa–stained cytospins were prepared on the indicated days to assess G-CSF–dependent morphologic maturation of 32D cells. The photographs depicted are representative of at least two independent experiments. (B) Cytospins from day 7 of G-CSF treatment from 32D/Neo and 32D/WRK cells were scored for neutrophil differentiation on the basis of morphology, and the results of three differential counts of 100 cells are each depicted with standard error. (C) 32D/Neo and 32D/WKR cells were analyzed by FACS after staining with anti-CD11b monoclonal antibody (thick line) or isotype control antibody (thin line) on the day before (day 0) and the indicated days after initiation of G-CSF treatment.