REL and Parkinson disease: In addition to these possible explanations to justify how c-Rel deficiency may lead to PD, relevant clues may also derive from the fact that this factor is a key regulator of immunity by affecting T-cell differentiation and regulatory T-cell (Treg) function (Hilliard et al., 2002; Visekruna et al., 2012; Grinberg-Bleyer et al., 2017; Luu et al., 2017).