Peptides derived from two regions of α-syn (Y39 and S129) produce immune responses in patients with PD which are enacted mostly by IL-5-secreting CD4+ T cells, as well as IFNγ -secreting CD8+ cytotoxic T cells. The Y39 antigenic region is strikingly close to the α-syn mutations that cause PD. The gene discussed is CD4; the disease is Parkinson disease.