Reduced CD8+ cytotoxic T cell numbers have been shown in AD compared with controls.23 However, other studies have shown no alterations in CD4+ or CD8+ T cell subsets in AD.24 25 B cell populations have been shown to be either reduced in AD and PD,22 25 or unchanged in AD.26 Inconsistent results in this field are likely due to differences in methodologies. Here, CD8A is linked to Parkinson disease.