T2D-risk variants at SLC30A8, TCF7L2, ADCY5, HNF1A, and MTNR1B consistently mapped to a cluster characterized by an association between T2D risk, reduced insulin levels, but elevated proinsulin levels, whereas those at ARAP1, IGFBP2, DGKB, and CCND2 combined T2D risk and reduced β-cell function with reduced proinsulin levels. This evidence concerns the gene HNF1A and type 2 diabetes mellitus.