The authors realised its potential application in the treatment of PD and a subsequent in vivo study in a mouse model of PD found that direct injection of GDNF into the substantia nigra or striatum resulted in a relative increase in dopaminergic nerve fibre density and improvements in motor behaviour regardless of whether the GDNF was administered before or after the MPTP used to induce parkinsonism [78]. This evidence concerns the gene GDNF and Parkinson disease.