Detection of gene transfer into day 12 spleen colony-forming cells using the GFP marker. Alternate, serial fresh frozen sections of spleens from animals transplanted with the GFP vector (A through D) or a control, CD24 vector (E and F) were stained with hematoxylin-eosin (A, C, and E) or directly visualized by fluorescence microscopy (B, D, and F) as described in Materials and Methods. In (A) and (B) the different arrows mark spleen hematopoietic colonies demonstrating a range of fluorescence intensities. In (C) and (D), the bold arrow denotes a GFP-marked megakaryocyte. Scale bars in (A) and (B) are equal to 325 μm, in (C) through (F) they are equal to 165 μm.