Megakaryocytic colonies grown in plasma clots from fetal liver cells of mice carrying a µLCR-pro-LacZ transgene. Note that X-gal staining is positive in megakaryocytes and some scattered erythroid cells (groups of 3 cells with bronze appearance because of benzidine co-staining), whereas background leukocytes are negative. See the article by Papayannopoulou et al, beginning on page 1274 of the February 15, 2000, issue of Blood.