The methylation levels of CpG sites within the GDNF promoter were measured and the results revealed a significantly higher percentage of methylated clones in the VTA of MD rats compared with NOR rats (t = 6.55, p < 0.001; Figure 3D) Concomitantly, The expression of the GDNF mRNA (t = −3.81, p = 0.003; Figure 3E) and protein (t = −2.61, p = 0.026; Figure 3F) in the VTA of MD rats were both significantly lower than that in NOR rats. Here, GDNF is linked to Menkes disease.