BACKGROUND AND OBJECTIVESThe prevalence of maturity-onset diabetes of the young (MODY) in Saudi population remains unknown, and data on molecular etiology of this condition is limited. Therefore, the present study was undertaken to elucidate clinical and molecular characteristics of a Saudi family with MODY 1.DESIGN AND SETTINGSThis is a case series study conducted at Saad Specialist Hospital in Alkhobar, Saudi Arabia.PATIENTS AND METHODSA 12-year-old female presented to us with symptoms suggestive of diabetes. Investigations revealed hyperglycemia, glycosuria, and ketonuria without acidosis. Pancreatic antibodies were negative. She responded well to subcutaneous insulin. Her family history revealed that 2 of her siblings were diagnosed with type 1 diabetes (T1DM), while her father and mother had type 2 diabetes (T2DM). In view of this strong family history, the possibility of monogenic diabetes was raised, and the 2 genes consistent with this phenotype, hepatocyte nuclear factor-1 alpha (HNF1α) and hepatocyte nuclear factor-4 alpha (HNF4α), were studied. Accordingly, genomic DNA was isolated from peripheral blood lymphocytes of the 8 members of this family, polymerase chain reaction was carried out, and sequencing of the whole HNF4α and HNF1α genes was done.RESULTSDNA study of the proband revealed a heterozygous substitution in intron 1 (IVS1b C>T-5)(c.50–5C>T) of the HNF1α gene. This mutation was identified in other 5 members of the family.CONCLUSIONThis study alerts physicians to suspect MODY in patients who have a strongly positive family history of diabetes over a few generations with negative pancreatic antibodies and absence of ketoacidosis and obesity.