RBD can manifest de novo as idiopathic RBD or can be associated with a heterogeneous group of conditions including narcolepsy, alpha-synuclein neurodegenerative disorders (especially Parkinson’s disease and dementia with Lewy bodies), paraneoplastic neurological syndromes, autoimmune disorders, central nervous system lesions (e.g., tumors, stroke), other neurological disorders, psychiatric disorders (post-traumatic stress disorders and mood disorders), antidepressant/other medications, drug withdrawal states, and toxic metabolic states. The gene discussed is SNCA; the disease is Lewy body dementia.