We describe siblings born from non-consanguineous parents, with older sibling having asymptomatic hyperglycaemia while younger sibling presented with low birth weight and persistent hyperglycaemia from first month of life. Our case, the older sibling was heterozygous for paternally inherited GCK pathogenic variant resulting in diabetes of maturity-onset in the young (MODY) manifested as mild fasting hyperglycaemia. GCK gene sequencing revealed that the younger sibling was compound heterozygous for missense mutations (two) combined in a novel GCK -permanent neonatal diabetes mellitus (PNDM) genotype. Thus, heterozygous inactivating GCK mutations are likely to lead to maturity-onset diabetes of the young type 2 (MODY 2) and the homozygous inactivating or compound heterozygous GCK mutations are a cause of PNDM.