Diabetes mellitus (DM) is a genetically heterogeneous disease, and some cases of type 1 diabetes mellitus (T1DM) are caused by a mutation of one gene. The child has suffered from brittle diabetes since the age of 1 year and received insulin. The HbA1c level is 9.9—11.4%. The patient was examined at the age of 14 years; the disease duration is 13 years. The daily dose of insulin is 1.15—1.35 U/kg. DM complications include initial manifestations of distal diabetic polyneuropathy as well as cheiropathy. The mother (39 years) and grandmother (74 years) have T1DM with multiple complications. A genetic study revealed that all three family members had a heterozygous substitution p.P274 in the PTF1A gene with an unknown pathological significance. At present, it can not be asserted that the identified mutation is the etiologic factor of diabetes in the described case. We can not exclude T1DM in this child because specific antibodies have not been tested. However, the mutation is detected in all three members of the same family with diabetes, which does not exclude the discovery of a new form of MODY, not described earlier.