Any process that modulates the frequency, rate or extent of gene expression; the process is mitotically or meiotically heritable, or is stably self-propagated in the cytoplasm of a resting cell, and does not entail a change in DNA sequence. [ http://www.ncbi.nlm.nih.gov/pubmed/11498582 http://www.ncbi.nlm.nih.gov/pubmed/10521337 ]