Scientists have so far isolated eight types of maturity-onset diabetes of the young (MODY). These uncommon forms of diabetes are caused by single-gene mutations that, in turn, affect the functions of the insulin-producing beta cells of the pancreas.

About 1 to 5 percent of all diabetes cases result from these genetic mutations, according to the American Diabetes Association. MODY can be broken down by the genetic defects causing the condition:

• MODY1 (HNF-4alpha gene - hepatic nuclear factor-4alpha). Although it was the first gene defect discovered, it is an extremely rare form of MODY with similar effects to MODY3. It can cause a severe form of diabetes that may need to be controlled with insulin.

• MODY2 (pancreatic glucokinase gene, GCK). In normal functions, this gene allows the body to monitor and adjust insulin secretion as glucose (blood sugar) levels fluctuate. However, when the gene is not functioning properly, the levels of blood glucose tend to rise. MODY2 normally does not show signs or symptoms but rather is detected during routine blood testing. The main treatment is changes in diet. It is most commonly diagnosed in childhood or during pregnancy.
  
  
• MODY3 (HNF-1alpha or hepatic transcription factor-1, TCF1). MODY3 occurs more frequently than other types of MODY. It is normally diagnosed later in life than the others. MODY3 can cause progressive diabetes, which may result in diabetic complications. People with MODY3 may be at an elevated risk of developing the eye disease diabetic retinopathy. Patients are often receptive to sulfonylureas, a class of antidiabetic agents. MODY 3 can cause a severe form of diabetes that may need to be controlled with insulin.
  
  
• MODY4 (insulin promoter factor-1 gene, IPF-1; or pancreas duodenum homeobox-1, PDX1). It is a rare form of MODY that often is associated with mild forms of diabetes.
  
  
• MODY5 (HNF-1beta gene): This gene is sometimes referred to as hepatocyte transcription factor-2 (TCF2). This rare form of MODY is often associated with renal disease, including kidney dysfunction or cysts, which are frequently diagnosed before the diabetes.

• MODY6 (neuroD1 gene): This is an extremely rare form of MODY. Little is known about the severity of diabetes associated with MODY6.
  
  
• MODY7 (KLF11 gene). Little is known about this form of MODY, first reported in 2005.
  
  
• MODY8 (carboxyl-ester lipase gene). First described in 2006, this form of MODY is said to involve early-onset diabetes and dysfunction of the exocrine pancreas (i.e., impairment in the production of digestive enzymes).

Some MODY patients will not present with one of these recognized gene mutations. Some physicians refer to this type as MODY X as researchers continue to search for the gene disorders responsible.

In addition, scientists have described a dominant mutation in the sulfonylurea receptor-1 gene (ABCC8) that causes hyperinsulinemia in infancy, impairs insulin secretion in early adulthood and leads to diabetes in middle age. This condition resembles MODY except for the later age of onset of the diabetes. Some researchers thus propose that the term “maturity-onset diabetes of the young” be replaced with the term “autosomal dominant type 2 diabetes.”