Genetic factors play an important and probably necessary role, in the development of most forms for diabetes mellitus, but are not the sole cause of the disease. The disease seems to be caused by an interaction between polygenic inheritance and environmental factors. In type I diabetes, more than ten different genetic markers have been found to be associated with the disease. Genes in the HLA-region, on and near the insulin gene, are the most important ones. In the common form of type II diabetes, a series of candidate genes which are code for important steps in insulin secretion or action have been investigated. So far, none have been identified that contribute significantly to the development of the disease in the majority of cases. The most significant breakthrough in research in recent years has been in the study of maturity-onset diabetes of the young (MODY). Until now, mutations in five different genes, which are code for glucokinase and various transcription factors, have been found to be associated with this form of diabetes. Knowledge of the genetic heterogeneity of diabetes has led to a need for a more pathophysiologically correct classification of the disease.