A range of studies employed flow cytometry to quantify and compare peripheral CD4+ T cell subtypes in PD, not only comparing healthy subjects, drug-treated and/or drug-naive PD patients (e.g. ref. [81]), but also analyzing immune cell subtypes patients at different disease stages (e.g. ref. [92, 93]) and with disease phenotypes (e.g., [90, 91, 94]. This evidence concerns the gene CD4 and Parkinson disease.