Over the past months, as this special issue took shape, the Editors of Science have monitored an exchange of seven tetters initiated by three queries from M. Bacon. These queries concern the popular definitions of "genes," "genetics," and "epigenetics." Below, we reprint excerpts from these letters, referring interested readers to www.sciencemag.org/cgi/ content/full/293/5532/1103/DC1 for the complete text and additional references.