TMPRSS6 and alopecia: To examine the effect of hypoxia on IRIDA (Figure1A), we generated global Tmprss6 knockout mice (Tmprss6−/−) (Figure S1A,B, Supporting Information), characterized by almost complete alopecia, which could be rescued by parenteral iron supplementation.[10, 12] Mice exposed to hypoxia (10% O2) for 4 weeks demonstrated a near complete hair recovery (Figure 1B).