Considering the age-dependent evolution of PD from prodromal disease in the ENS to full motor manifestations following midbrain degeneration, Cdnf−/− mice do not exhibit a significantly enhanced age-related decline in neuronal density in the MyP compared to that of Cdnf+/+ mice at any age examined (from 1.5- to 12-month old). The gene discussed is CDNF; the disease is Parkinson disease.