GCK and Hyperglycemia: The GCK phenotype is strikingly similar between patients; fasting hyperglycaemia in the range of 5.5–8 mmol/L, HbA1c 40–60 mmol/L and a small postprandial increase in glucose (typically <4 mmol/L difference between fasting and 2 h values on OGTT) because the expression of the unaffected gene copy is upregulated to compensate for the loss the activity of the defective gene.36