In 2003, researchers studying inherited PD discovered that the disease in one large family was caused by a triplication of the normal alpha-synuclein gene on one copy of chromosome 4 (a chromosome is a threadlike structure of a protein and the genetic material DNA). This triplication caused people in the affected family to produce too much of the normal alpha-synuclein. This study showed that an excess of the normal form of synuclein could result in PD, just as the abnormal form does.