The age at diagnosis of diabetes was higher in HNF4α- and HNF1β-MODY probands than in the other MODY types, including MODY X. MODY X probands exhibited higher fasting glucose levels and glucose levels at 60 min during an OGTT compared with those of GCK-, HNF1α-, and HNF4α-MODY probands. This evidence concerns the gene HNF1A and diabetes mellitus.