The unique nucleotide sequence of the A beta gene in the NOD mouse is shared with its nondiabetic sister strains, the ILI and the CTS mouse.

It is generally held that one of the recessive genes controlling diabetes in the NOD mouse is linked to the major histocompatibility complex (MHC). Unique substitution of Asp57 with Ser in the A beta chain is considered to make the A beta gene the MHC-linked susceptibility gene. We therefore analysed the nucleotide sequences of the A beta second exon in ILI, CTS, and NON mice, which are nondiabetic inbred strains but are derived from the same Jcl-ICR mice as the NOD mouse. The DNA sequence analyses revealed that the A beta second exon sequences in the ILI and CTS mice, but not in the NON mouse, are identical to that of the NOD mouse. Possible roles of Ser57 of the A beta chain in the nondiabetic sister strains are discussed.