Hereditary forms of iron-deficiency anemia, including animal models, have taught us much about the normal physiologic control of iron metabolism. However, the discovery of new informative mutants is limited by the natural mutation frequency. To address this limitation, we have developed a screen for heritable abnormalities of red blood cell morphology in mice with single-nucleotide changes induced by the chemical mutagen ethylnitrosourea (ENU). We now describe the first strain, fragile-red, w...