Ankyrin deficiency is one of the most common causes of hereditary spherocytosis in humans. A spontaneous mutation, normoblastosis (Ank1nb), discovered in 1969 in a mouse stock maintained at the Jackson Laboratory, provides an important animal model for these human ankyrin-deficient anemias. Study of this model has led to the finding of multiple isoforms of Ank1 as well as Ank1nb-related pathology in nonerythroid tissues. To enhance the usefulness of this model, we have identified the Ank1nb mutation as the deletion of a guanosine residue in exon 36 of the erythroid ankyrin gene (Ank1). This results in a frame shift that introduces a stop 13 codons downstream and predicts a 157 kDa nb-ankyrin lacking the regulatory domain but including intact membrane- and spectrin-binding domains. By epitope scanning on immunoblots, we show that a previously reported protein (p150) found in nb reticulocytes is the predicted nb-ankyrin. Existing evidence indicates that this protein is functional, making the normoblastosis mutation a hypomorph rather than a null as originally thought. The nb-ankyrin provides an explanation for the milder phenotype displayed by nb/nb animals relative to the murine spectrin-deficient anemias, spherocytosis (Spna1(sph), Spna1(sph-1J), Spna1(sph-2BC), Spna1(sph-DEM)) and jaundiced (Spnb1(ja)), and suggests that truncated ankyrins could be useful in gene replacement therapy.