Iron transport via Mfrn1 is required for erythroid cell cycle progression. (A) Mfrn1 deficiency does not cause increased erythroid cell death. We did not observe a significant difference between WT, frs/+, and frs/frs groups in the numbers of cells with sub-G0 DNA. (B) Most WT erythroid cells were in G0/G1, suggesting they were quiescent. There was a significant decrease in the numbers of frs/+ and frs/frs erythroid cells in G0/G1. Addition of supplemental iron significantly increased the percentage of frs/frs erythroid cells in G0/G1. (C) There were no significant differences between groups in the percentage of erythroid cells in S phase. (D) frs/+ and frs/frs mutants had a significant increase in the percentage of erythroid cells arrested in G2/M. Addition of supplemental iron significantly reduced the percentage of mutant erythroid cells in G2/M arrest. However, even iron supplementation could not reduce the percentage of erythroid cells in G2/M to WT levels. (E) Mfrn1 deficiency did not significantly affect cell death in nonerythroid (GFP⁻) cells. (F) Mfrn1 deficiency significantly decreased the proportion of nonerythroid cells in G0/G1, suggesting an increase in the number of cells exiting quiescence. Supplemental iron restored the proportion of cells in G0/G1 to WT levels. (G) There were no significant differences between groups in the percentage of erythroid cells in S phase. (H) frs/frs mutants had a significant increase in the percentage of nonerythroid cells arrested in G2/M. Addition of supplemental iron significantly reduced the percentage of frs/frs nonerythroid cells in G2/M arrest to WT levels. ∗∗P < .01; ∗∗∗P < .001; ∗∗ and ∗∗∗ obtained for pairs with 95% significance.