HNF1A and maturity-onset diabetes of the young type 3: use human MODY3 patient and CRISPR/Cas9 edited iPSC lines to show that the HNF1αp291fsinsC mutation results in the expression of a mutant truncated HNF1α protein that interacts with wild-type HNF1β protein and inhibits its function, impairing pancreatic progenitor and β cell differentiation.