STAT5 activation is required for Friend virus-induced polycythemia. Hematocrits of different strains of mice are shown at baseline (pre-FVP) and 3 to 4 weeks after infection with FVP (post-FVP). The mouse strains and genotypes are indicated at the top. In each panel, the pre-FVP hematocrits are shown on the left and the post-FVP hematocrits on the right with a line connecting the values. Low hematocrit values were occasionally caused by internal bleeding.