Histologic studies. Blood smear from a 1-month-old wild-type (A) or EPAS1/HIF-2α null (B) mouse. Bone marrow sample preparations from a 1-month-old wild-type (C) or EPAS1/HIF-2α null (D-F) mouse stained with Giemsa stain (C-D) or assayed for β-galactosidase activity (E-F). The samples stained for β-galactosidase activity demonstrate examples of positively stained vascular endothelial cells (E), bone-lining cells (F), and possibly other adventitial cells (E-F). WT indicates wild-type; and KO, EPAS1/HIF-2α null. Original magnifications, × 100 (A-B) and × 20 (C-F).