We report a 21 years old woman, without offspring, with diabetes mellitus diagnosed at 17 years of age, without ketosis or weight loss. Her body mass index was 18 kg/m2. Her C peptide was normal (2.3 ng/ml) and diabetes mellitus type 1 autoantibodies were negative. A monogenic diabetes Maturity Onset Diabetes of the Young (MODY) was proposed. Her family study disclosed a diabetic father and a brother with altered fasting glucose levels. The University of Exeter score for MODY yielded a 75.5% probability of MODY2. In the genetic-molecular study of the glucokinase gene (MODY2), the patient had a mutation at position 1343 of exon 10, corresponding to a heterozygous substitution of guanine by adenine (1343 G >A). The same mutation was found in her father and brother. This mutation is different from those previously described in the literature. The described change determines that a glycine is replaced by aspartic at amino acid 448 of the enzyme (non-synonymous substitution). The diagnosis of MODY2 was therefore confirmed in the patient and her father. The mutation was inherited by paternal line.