GCK and type 2 diabetes mellitus: Conversely, variants that decrease activity are linked to diabetes: GCK-maturity-onset diabetes of the young (GCK-MODY, MIM# 125851) when heterozygous [10, 11] and permanent neonatal diabetes mellitus (PNDM, MIM# 606176) if homozygous or compound heterozygous [12, 13].