Morphologic differentiation of transfectants in response to G-CSF. Cytokine-treated cells were photographed 48 hours after addition of IL-3 or G-CSF and after May-Grünwald-Giemsa staining. Original magnification × 200. (A) BaF3 cells transfected with vector alone retain the immature morphologic appearance of untransfected BaF3 cells. (B) Ba/F3 cells transfected with G-CSFR have a similar immature morphology when grown in IL-3. (C) When grown in G-CSF, Ba/F3GR cells acquire morphologic features of myeloid differentiation. (D) Ba/F3 cells transfected with the GRprox mutant, however, show little evidence of differentiation in response to G-CSF. (E) The appearance of myeloid differentiation is accentuated when Gab2 is overexpressed in cells expressing full-length G-CSFR. (F) Overexpression of Gab2 in GRprox cells partially restores the morphologic effects of the wild-type receptor. (G) Cells expressing a mutated Y764F receptor did not show differentiation.