TPO treatment (100 μg/kg per day for 5 days) increased the frequency of Mks and CFU-Mks in hematopoietic tissues from both wild-type and GATA-1^low littermates. Frequency of Mks (A) and CFU-Mks (B) in marrow (▪), spleen (□), and liver (▦) from wild-type and GATA-1^low mice at days 7, 10, 14, and 21 after receiving 100 μg TPO/kg of body weight/day for 5 consecutive days. The data are reported both in absolute number (top) and as percent change, with respect to values measured in each mouse 3 to 5 days before TPO treatment (bottom). Results represent the mean ± SD of data obtained on 4 mice at each time point. The numbers of Mks were determined in serial hematoxylineosin–stained sections similar to those presented in Figure 2. The numbers of CFU-Mks are expressed per 10⁵ mononuclear cells (MNCs) for all the tissues. Values statistically higher than those observed in untreated animals: ^* indicates P < .05; §, P < .01.