We focused on the role of modification of DNA methylation in several genes associated with depression (NR3C1, NR3C2, CRHR1, SLC6A4, BDNF, and FKBP5), Rett syndrome (MECP2), Alzheimer's, depression (APP, BACE1, BIN1 or ANK1) and Parkinson's disease (SNCA), as well as the co-occurring modifications to histones and expression of non-coding RNAs. The gene discussed is SLC6A4; the disease is Parkinson disease.