In 2005, Toye et al. [12] found, using a classical genetic mapping approach, that glucose intolerance in C57BL/6J is largely due to a naturally occurring “knock-out” of the Nnt gene, encoding nicotinamide nucleotide transhydrogenase, a nuclear-encoded mitochondrial proton pump [49]. The gene discussed is NNT; the disease is Glucose intolerance.