β-Gal expressing ROSA 26 donor cell engraftment in the lung and brain of B6/129 F2 animals 1 month after transplantation. Only rare β-gal–expressing cells are found in the lung (A) or brain (B) of B6/129 F2 animals by 1 month following transplantation with ROSA 26 BM. The positive cells shown in (B) are seen in a leptomeningeal site. No positive cell was seen in any site in tissues from animals transplanted with control BM. These pictures were selected to document the few positive cells at these sites.