β-Gal histochemical staining of cryostat sections of spleens from B6/129 F2 animals 1 month after transplantation with ROSA 26 or B6/129 F2 BM. None of the cells in the spleen of a B6/129 F2 mouse transplanted with control B6/129 F2 marrow stain blue indicating the absence of β-gal activity (A). Nearly all cells in the spleen of a B6/129 F2 animal transplanted with ROSA 26 marrow stain blue indicating engraftment with β-gal expressing ROSA 26 BM (B). Some positive cells are highlighted by arrows.