INS and Impaired glucose tolerance: Although studies have suggested that mutations in the nuclear encoded nicotinamide nucleotide transhydrogenase (Nnt) gene in the C57BL/6J relative to the C57BL/6N are associated with changes in insulin secretion and impaired glucose tolerance in the C57BL/6J mouse (30, 53), others have questioned differences that appear to exist when performing in vitro versus in vivo studies (54); however, both the C57 control and MNX (harboring the C3H mtDNA) mice in this study shared the same nuclear genetic background.