CFU-GM colony growth from Mx1-Cre, Nf1^flox/flox and Cre^- mice. (A-B) CFU-GM colony growth at various concentrations of GM-CSF. Bone marrow mononuclear cells were plated in duplicate in methylcellulose. Cre⁺ bone marrow from mice with MPD show a left shift in the GM-CSF dose-response curve when expressed in terms of total numbers of colonies (A) or the calculated percentage of maximal colony growth (B). Colony numbers in duplicate 1-mL plates are shown with ranges from a representative experiment. (C-D) CFU-GM colonies grown from Mx1-Cre, Nf1^flox/flox and control mice photographed at 40 × magnification. A typical CFU-GM morphology from a normal mouse is shown in panel C at 40 × magnification. The colonies grown from Cre⁺ mice with MPD are larger and show abnormal spreading (D). (E-F) Cytospins of CFU-GM colonies stained with Wright-Giemsa from a wild-type mouse (E) contain approximately 70% neutrophils compared with 93% monocyte-macrophage cells in Cre⁺ mice. Original magnification, × 500 for panels E and F.