CDKN1A and Parkinson disease: Blood-based expression of these markers has not been well-studied in the context of neurodegeneration, although p21 expression in monocytes has been reported to be lower in Alzheimer’s disease [41], and we have previously found evidence of reduced p21 expression in total leukocytes in early PD, as well as an association between lower leukocyte p16 expression and more rapid disease progression [30].