A marked increase in ICAM-1 was found within the SN at sites of T cell infiltration following MPTP-intoxication[104] and numbers of peripheral CD4+ T cells that expressed α4β7 integrin were diminished and inversely correlated to clinical severity in PD patients[102] suggesting that Teffs are removed from the peripheral circulation, either by elimination or dissemination to sites of CNS inflammation, and may play a role in disease progression. This evidence concerns the gene CD4 and Parkinson disease.