Histologic analysis of the myelofibrotic trait in 12-month-old heterozygous F1 Gata1^low females carrying the mutation in a predominantly CD1, C57BL/6, or DBA/2 background. Blood smears are shown on the right panels, while hematoxilin-eosin and Gomori staining of liver, spleen, and marrow are shown on the top and bottom panels, as indicated. The genetic background of the mice analyzed is specified in Figure 1, while the quantification of Mk frequency and of the fibrosis in each animal group is presented in Table 2. The arrow in the liver section in panel A indicates an erythroid nest within the parenchyma, already detectable at 12 months in CD1 mutants, while the arrowheads on the blood smears in panel C indicate clusters of normal platelets, that is, deriving from the stem cell population that had inactivated the X chromosome carrying the Gata1^low allele. These clusters were detected at high frequency, mainly on blood smears of heterozygous DBA/2 mutants. Similar results were obtained in at least 3 to 6 animals for experimental points. Original magnifications are indicated at the bottom of each panel.