This allele was first described as having four point mutations: A82T, V87A, S258L, and D301N. The first two mutations are in the most N-terminal of the four homologous repeats of the stalk region and are predicted to form an alpha-helical coiled-coil arrangement on trimerisation. S258L is in a region essential for IgE binding. A later study revealed a fifth point mutation, K131E, that occurs in the stalk region. This allele is found in mice of 129 or NZB origin.