Narcolepsy usually starts in teenagers or young adults. Both men and women are affected equally. In most cases, excessive daytime sleepiness is the first symptom and cataplexy develops later. In very few cases, narcolepsy is familial, where 2 or more members of the same family affected. Overall, narcolepsy has a low heritability (narcolepsy is rarely transmitted from an affected person to his/her children) and families with several patients are very rare. Nevertheless, there are several genetic risk factors in narcolepsy. The most important one is a specific variant of an immune-related gene called HLA-DQB1. Nearly 95% of narcolepsy patients carry the HLA-DQB1*0602, but the gene is not abnormal and 15-25% of healthy subjects carry the same gene. Recent research indicates that other immune-related genes contribute to the development of narcolepsy. As of today, no causal genetic evidence is available.