To clarify the contribution of Cblb to the development of type1 diabetes (T1D), we investigated Japanese younger-onset T1D patients. We sequenced the cblb gene in 10 T1D patients and screened the identified mutations in 109 Japanese T1D patients and 100 normal subjects. In addition to four previously reported synonymous single nucleotide polymorphisms (SNPs), we identified two novel nonsynonymous variants (786 C>T (A155V) and 1718 A>G (N466D)). The A155V mutation was found in one subject with Basedow's disease whose mother also carried both the mutation and Basedow's disease. The N466D mutation was found in 6 T1D cases including a subject who was classified as fulminant T1D. We found no significant differences in the allele frequency of these SNPs among T1D and control subjects, suggesting that the contribution of cblb to the genetic susceptibility to T1D might not be high for Japanese younger-onset T1D.