hem6 reticulocytes have a defect in iron incorporation into heme partially corrected by ALA. (A) Reticulocyte iron uptake and heme biosynthesis were measured in 129B6N₂F₁hem6/hem6 (n = 4; ■) and +/+ (n = 4; ▩) control littermates following 30 minutes of incubation with ⁵⁵Fe₂-transferrin with or without 2 mM added ALA. The data are normalized to and presented as uptake per microgram of RNA per hour. (B) Average heme iron to whole-cell iron ratio from 3 independent experiments with or without ALA treatment is shown. Error bars represent plus or minus 1 SD.