To date, KOR receptor gene (OPRK1) was the first found epigenetically modulated at chromatin levels by stress and nerve growth factor (NGF) [122]; in P19 mouse embryonal carcinoma cells, mechanisms involving chromatin changes in promoter region (i.e., demethylation at Lys9 and dimethylation at Lys4 of histone H3), transcription factor activation protein 2beta (AP2b) and PI3K system were reported [122]; in hippocampal cell lines Ht22, involvement of transcription factor Myc has been also demonstrated [123]. This evidence concerns the gene NGF and embryonal carcinoma.