Introduction: Maturity Onset Diabetes of the Young (MODY) is a group of disorders responsible for development of monogenic diabetes mellitus (DM) characterized by autosomal dominant inheritance. MODY 2, caused by an inactivating mutation of the GCK gene, in its heterozygous form causes mild hyperglycemia that often does not require an hypoglycemic therapy due to the low risk of developing DM complications. Case presentation: A 20-year-old caucasian woman was referred to our center due to worsening glycemic control, although hematological examinations were not yet diagnostic for DM (fasting plasma glucose 121 mg/dl and HbA1c 6.6%). The patient reported mild fasting hyperglycemia since the age of 14, low birth weight, a family history of DM on the paternal side, denied chronic therapy intake, was of normal weight and investigations demonstrated the absence of beta-cell autoimmunity. In the suspicion of MODY, the patient underwent genetic analysis. A synonymous substitution in the GCK, the c.579G>T (p.Gly193=) was detected. It has been already described in Italian cases of GCK/MODY and an alteration of the splicing has been supposed. However, to date, no functional study had been conducted to verify the pathogenic role of the mutation. The use of the minigene strategy made it possible to confirm that this mutation results in the loss of the canonical donor splicing site of exon 5 with generation of a novel one leading to the loss of the last two nucleotides of the exon 5. The altered splicing causes a frameshift in the GCK coding sequence and insertion of a premature termination codon. Conclusion: The functional genetic study performed has demonstrated the causative role of the synonymous c.579G>T (p.Gly193=) variant in the GCK gene in the development of MODY2.