Work on animal models of PD has demonstrated that part of the neuroprotection afforded by endurance exercise is attributable to BDNF and GDNF, two neurotrophic factors that are reduced in the SNpc of human PD patients by as much as 20% per neuron, relative to healthy age-matched controls (Chauhan et al., 2001). The gene discussed is GDNF; the disease is Parkinson disease.