It is thought that most mental disorders have a genetic component. A study on a large family in 1993 identified a link between a particular type of mental retardation, which included frequent aggressive and violent outbursts, with a region on the X chromosome in some of the males. This is one of the few cases in which a direct correlation between a single gene defect and a particular type of mental disorder is apparent. But in 2013, National Institutes of Health-funded researchers discovered that people with disorders traditionally thought to be distinct—mainly autism, ADHD (Attention Deficit Hyperactivity Disorder), bipolar disorder, major depression, and schizophrenia—were more likely to have a possible genetic variation at the same four chromosomal sites. More studies will need to be done to verify these genetic possibilities.