In summary, we describe a novel pathogenic MAFA missense variant that is highly likely to cause impaired phosphorylation and, therefore, functional defects, cosegregating with both phenotypes, insulinomatosis and MODY-like diabetes (“MODY-like”: autosomal dominantly inherited, rather mild hyperglycemia (diabetes/IFG/HbA1c > 5.6) caused by a pathogenic variant in a transcription factor regulating insulin secretion). This evidence concerns the gene INS and diabetes mellitus.