The anemia of mutant homozygote C57BL/6SlmSlm mice is due to a gastrointestinal bleeding defect and to an erythropoietic defect. The anemic animals lose approximately 3.5 per cent of their blood volume into the gastrointestinal tract daily. The cause of this blood loss has not been established. The anemic animals also have an erythropoietic defect which renders them unable to respond normally to erythropoietin or to hypoxia. Although the homozygotes appear refractory to erythropoietin in moderately large single doses, they do respond if sufficiently large doses are given repeatedly.

The erythropoietic defect detected in the homozygote is also evident in the heterozygote. However, the defect remains latent in the heterozygote, for although this animal is subnormally responsive to erythropoietin and to hypoxia, it is not anemic.

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