GFP expression in hematopoietic colonies from mice transgenic for construct 3. Cells were observed in the visible (i) and fluorescent (ii) light at day 4 to 5 of culture (A) and day 9 to 12 (B, CFU-mix; C-E, BFU-E; F, CFU-mix). Original magnification, × 100. Similar results are obtained also with construct 2, both using fetal liver and marrow cells. No fluorescence is seen with cells from construct 1 mice, and from nontransgenic controls. Arrows in panel F point to a group of myelomonocytic cells expressing little or no GFP.