Given that the proband had early-onset diabetes and tested negative for diabetes-related antibodies (GAD, IA2, IAA, ICA, and Zn-T8), exome sequencing was performed to identify the D260N mutation in HNF4A. The proband’s father, at the age of 27, was diagnosed with diabetes through an oral glucose tolerance test (OGTT) and was confirmed to be a carrier of the D260N mutation. Here, HNF4A is linked to diabetes mellitus.