Mx-Cre–mediated deletion of the Gata1 gene in adult mice affects erythropoiesis. (A) Top, the mouse Gata1 locus. Middle, floxed Gata1 allele generated by inserting 2 loxP sites and the splice acceptor (SA)/GFP cDNA cassette. Bottom, Gata1 null allele obtained after Cre-mediated recombination. IT, IE, and II-VI indicate exons of Gata1 gene. E and S indicate EcoRI and SalI restriction sites, respectively. Positions of probes used for Southern blot analysis and PCR primers (forward, reverse, reverse GFP) are indicated. (B) Peripheral blood counts of Gata1^fl/y::Mx-Cre(+) mice (KO-Mx, ■) and controls (□) at 0, 1.5, and 3 weeks after pIpC injection. Results are shown as mean plus or minus SD from 4 mice: (i) white blood cell counts, (ii) red blood cell counts, (iii) hematocrit, and (iv) platelet counts (*P < .05 for KO-Mx mice compared with controls). (C) Frequency (%) of reticulocyte counts in peripheral blood from KO-Mx mice (■) and controls (□) at 0, 1.5, and 3 weeks after pIpC injection. Results are shown as mean plus or minus SD (error bars) from 3 mice (*P < .05 for KO-Mx mice compared with controls). (D) FACS analysis of BM cells prepared from the Gata1 BAC GFP (top) and the KO-Mx mice (bottom) at 3 weeks after pIpC injection. Expression of the erythroid surface markers Ter119 and CD71. Expression was examined in cell fractions gated for GFP⁺ cells. RI, RII, RIII, and RIV represent Ter119^lowCD71^high, Ter119^highCD71^high, Ter119^highCD71^med, and Ter119^high-CD71^low populations, respectively.²⁴  The relative number in each region as a percentage of gated cells is indicated. (E) Morphology of GFP⁺ and GFP⁻ cells isolated from the KO-Mx BM. GFP⁺ and GFP⁻ cells were sorted from RI, RII, and RIII fractions of the KO-Mx BM. Representative examples of Wright-Giemsa-stained cytospins are shown. High-resolution images were taken from 4 separate fields of view to show cellular morphology. Scale bar represents 100 μm. (F) Real-time RT-PCR analyses of Gata1, Gata2, and β-major globin mRNAs in regions I and II cells prepared from the GFP⁺ fraction of KO-Mx or control BM. BM cells from 3 mice/group were pooled and sorted by FACS. Data were normalized with glyceraldehyde-3 phosphate dehydrogenase (GAPDH) mRNA levels. The expression level of region I from control mice was shown as 1.0. Results are shown as mean plus or minus SD (error bars) from 3 samples.