The past few years have seen an explosion of heroin abuse and deaths from opiate overdose. But little is known about the molecular underpinnings of heroin addiction. A new study has found that heroin use is associated with excessive histone acetylation, an epigenetic process that regulates gene expression. More years of drug use correlated with higher levels of hyperacetylation. The study provides the first direct evidence of opiate-related epigenetic alterations in the human brain.