⁵⁹Fe incorporation into heme by cultured erythroblasts from conditional bcl-x^-/- mice and bled controls. At the indicated times, ⁵⁹Fe-transferrin was added to quadruplicate cultures of erythroblasts from conditional bcl-x^-/- mice or from bled control mice. After 2 hours of further incubation, incorporation of ⁵⁹Fe into heme was quantified for each labeled culture and presented as counts per minute per 10⁶ viable cells. Data are mean ± SEM of the quadruplicate cultures. The experiment shown is representative of 2 complete time-course experiments with 2 sets of erythroblast populations from conditional bcl-x^-/- and bled control mice. For each of the 6 time points in the 2 independent experiments, the cells from bled controls incorporated significantly more ⁵⁹Fe into heme per 10⁶ viable cells than the bcl-x^-/- erythroblasts.