Retroviral expression of AML1b rescues hematopoiesis from AML1-deficient P-Sp regions. Photographs were taken with a Nikon Eclipse TE2000-U (Nikon Sankei, Tokyo, Japan) at a magnification of × 100 after 5 days of culture (A-B) and 14 days of culture (C-D). (A) Hematopoietic cells emerged at day 5 from wild-type P-Sp regions. (B) No hematopoietic cells were observed in the culture of AML1-deficient P-Sp regions, showing only the background of OP9 cells. (C) AML1b-transduced P-Sp regions from an AML1-deficient embryo generated numerous round, nonadherent, or semiadherent cells. (D) A control culture infected with mock virus failed to generate any hematopoietic cells.