β-Gal Histochemical staining of cryostat sections of tissues from ROSA 26 animals. Frozen sections of tissues were incubated in the presence of X-gal and counterstained with safranin-O. All nucleated ROSA 26 BM cells stain blue, indicating the presence of β-gal activity (A). Most ROSA 26 cells stain with a bright blue perinuclear area. Other cells, such as the megakaryocyte (open arrow) stain with a more diffuse pattern. No cells in the control B6/129 F2 marrow stain blue, indicating the absence of β-gal activity (B). All nucleated cells in the adult ROSA 26 brain (C) and liver (D) also stain blue demonstrating β-gal activity. (B) is counterstained more intensely to allow the identification of individual cells.