In MODY3, HNF1A Single Nucleotide Polymorphism (SNPs) act in the development of the disease already in heterozygosis by the need for sufficient levels of protein with a dominant negative effect of the mutant allele [8,9], whereas, in the T2D and GD susceptibility, HNF1A SNPs act in homozygosity, only partially affecting HNF1A function [10]. This evidence concerns the gene HNF1A and type 2 diabetes mellitus.