Quantitative and functional deficiencies of the MRC have been implicated in a wide range of health and disease states, including mitochondrial disorders [2], neurodegenerative diseases such as Parkinson’s disease (PD) [3, 4], Alzheimer’s disease (AD) [5], and amyotrophic lateral sclerosis (ALS) [6], as well as in aging [7, 8]. The gene discussed is CD200; the disease is Parkinson disease.