 ΔFosB (DeltaFosB) – a gene transcription factor – overexpression in the D1-type medium spiny neurons of the nucleus accumbens is the "crucial" common factor among virtually all forms of addiction (i.e., behavioral addictions and drug addictions) that induces addiction-related behavior and neural plasticity.In particular, ΔFosB promotes self-administration, reward sensitization, and reward cross-sensitization effects among specific addictive drugs and behaviors.Certain epigenetic modifications of histone protein tails (i.e., histone modifications) in specific regions of the brain are also known to play a crucial role in the molecular basis of addictions.