Most genes cause disease when the function of at least one copy of the gene is disrupted (e.g., loss-of-function mutations) or compromised (e.g., missense mutations). Gain of function, resulting from increased dose of a gene product, is a less common mechanism of disease causation. Neurologists would be familiar with Charcot-Marie-Tooth disease type 1A caused by duplications of the PMP22 gene. Gene duplications are also well known in neurodegenerative diseases, such as duplications of the amyloid precursor protein gene (APP), resulting in early-onset Alzheimer dementia (AD) and cerebral amyloid angiopathy (CAA), and copy number increases of the synuclein-alpha gene, SNCA, causing dementia with Lewy bodies or Parkinson disease dementia.

当至少一个基因拷贝的功能被破坏（例如，功能丧失突变）或受损（例如，错义突变）时，大多数基因都会引起疾病。由于基因产物剂量增加而导致的功能获得是一种不太常见的致病机制。神经科医生应该熟悉由PMP22基因重复引起的 1A 型腓骨肌萎缩症。基因重复在神经退行性疾病中也很常见，例如淀粉样前体蛋白基因 ( APP ) 的重复导致早发性阿尔茨海默氏痴呆 (AD) 和脑淀粉样血管病 (CAA)，以及突触核蛋白-α 的拷贝数增加基因SNCA ，导致路易体痴呆或帕金森病痴呆。