Examination of EGFP expression in individual GM-CFU methylcellulose colonies by inverted fluorescence microscopy. Murine BM cells were transduced with the MFG-EGFP retroviral vector and subsequently analyzed for the presence of in vitro clonogenic progenitor cells. Individual GM-CFU were then examined with an Olympus IX70 inverted fluorescence microscope. Phase-contrast (A, C) and fluorescence micrographs (B, D) were prepared of an EGFP-negative (A, B) and an EGFP-positive GM-CFU (C, D); original magnification × 100.