Symptoms of narcolepsy can sometimes be seen during the course of a neurological disease process. In such instances, the term "symptomatic narcolepsy" is used, implying that the narcolepsy is a symptom of the underlying process rather than being idiopathic. Prader-Willi syndrome (PWS) is characterized by infantile hypotonia and failure to thrive, hyperphagia with early childhood obesity, hypogonadism, temperature instability, and developmental delay. Subjects with Niemann-Pick type C (NPC) disease have been reported to frequently display narcolepsy-like symptoms, including cataplexy. In Huntington's disease, disrupted hypocretin transmission was first suggested through the study of R6/2 mice, a murine model of Huntington's disease with accelerated disease progression. Further studies of symptomatic narcolepsy associated with inherited diseases are thus helpful to understand the pathophysiological mechanisms, especially in relation to the hypocretin impairments, for occurrence of excessive daytime sleepiness (EDS) and cataplexy.