Demonstration of EGFP expression in individual isolated spleen colonies and GM-CFU derived thereof. Mice were injected with MFG-EGFP–transduced murine BM cells and 13 days later individual spleen colonies were dissected. The viable cells were analyzed by flow cytometry and cultured in standard colony assays for the presence of in vitro clonogenic progenitor cells. Examples are shown of an EGFP-negative (A) and EGFP-positive (B) spleen colony as well as a representative GM-CFU derived from these colonies (C and D, respectively). Percentages of cells positive for EGFP expression are indicated.