After phenylhydrazine-induced stress, the expression of p27 protein in early erythroid precursors from Cul4A^+/– mice is elevated compared to wild-type controls. Wild-type and Cul4A^+/– mice were treated with phenylhydrazine as described for Figure 2. On day 4, bone marrow cells were isolated separately from each mouse, and proerythroblasts and basophilic erythroblasts were identified and isolated with a fluorescence-activated cell sorter, as described in Figure 1. Cul4A, p27, and β-actin protein levels were determined by immunoblot. Actin was used as a loading control. A representative immunoblot from 2 independent experiments and their combined results are shown (mean ± SEM for 5 wild-type and 6 Cul4A^+/– mice, *P = .03).