A series of recent studies in humans and the NOD mouse model have highlighted the central role that autoimmunity directed against insulin, in particular the insulin B chain 9-23 peptide, may play in the pathogenesis of type 1 diabetes. Both pathogenic and protective T-cell clones recognizing the B:9-23 peptide have been produced. This report describes the successful creation of BDC12-4.1 T-cell receptor (TCR) transgenic mice with spontaneous insulitis in F1 mice (FVB × NOD) and spontaneous diabetes in NOD.RAG^−/− (backcross 1 generation). Disease progression is heterogeneous and is modified by a series of genetic factors including heterozygosity (H-2^g7/H-2^q) versus homozygosity for H-2^g7, the presence of additional T-/B-cell receptor–rearranged genes (RAG⁺ versus RAG^−/−), and the insulin 2 gene knockout (the insulin gene expressed in the NOD thymus). Despite lymphopenia, 40% of H-2^g7/g7 BDC12-4.1 TCR⁺ RAG^−/− Ins2^−/− mice are diabetic by 10 weeks of age. As few as 13,500 transgenic T-cells from a diabetic TCR⁺ RAG^−/− mouse can transfer diabetes to an NOD.scid mouse. The current study demonstrates that the BDC12-4.1 TCR is sufficient to cause diabetes at NOD backcross 1, bypassing polygenic inhibition of insulitis and diabetogenesis.