Editing Downregulation and upregulation (section)

==Drug addiction==
Family-based, adoption, and twin studies have indicated that there is a strong (50%) heritable component to vulnerability to substance abuse addiction.<ref name="Walker2018">{{cite book |last1=Walker |first1=Deena M. |last2=Nestler |first2=Eric J. |chapter=Neuroepigenetics and addiction |title=Handbook of Clinical Neurology |date=2018 |volume=148 |pages=747–765 |doi=10.1016/B978-0-444-64076-5.00048-X |pmid=29478612 |pmc=5868351 |isbn=9780444640765 }}</ref>

Especially among genetically vulnerable individuals, repeated exposure to a drug of abuse in adolescence or adulthood causes addiction by inducing stable downregulation or upregulation in expression of specific genes and [[microRNA]]s through [[epigenetics|epigenetic alterations]].<ref name=Nestler2014>{{cite journal |last1=Nestler |first1=Eric J. |title=Epigenetic mechanisms of drug addiction |journal=Neuropharmacology |volume=76 Pt B |pages=259–68 |date=January 2014 |pmid=23643695 |pmc=3766384 |doi=10.1016/j.neuropharm.2013.04.004 }}</ref> Such downregulation or upregulation has been shown to occur in the brain's reward regions, such as the [[nucleus accumbens]].<ref name=Nestler2014 />