In the retina of PD patients, there were dopaminergic deficiency (Schmidt et al., 2011; Vecino et al., 2016), misfolded a-synuclein (Weil et al., 2016), retinal ganglion cells loss (Malmfors, 1963), thinning of retinal nerve fiber layer (Frederick et al., 1982; Hillman et al., 1995; Cameron et al., 2009; Zhang et al., 2014), or neuroinflammatory (Cohen et al., 1992) at several levels of the visual pathway during pre-clinical stages. The gene discussed is SNCA; the disease is Parkinson disease.