HNF1A and maturity-onset diabetes of the young type 3: Interestingly, HNF1αp291fsinsC phenotypes in MODY3 have been attributed to a dominant negative effect due to the formation of a truncated HNF1α protein (Okita et al., 1999; Vaxillaire et al., 1999; Yamagata et al., 1998) or haploinsufficiency mediated through nonsense-mediated decay (Harries et al., 2004).