Formaldehyde’s toxicity was presumed to derive from its ability to crosslink DNA, but that happens only at huge doses. Recent discoveries that formaldehyde is found in the body in small amounts led a team to investigate its natural role. They found that formaldehyde regulates epigenetic modification of DNA, interfering with an enzyme that carries methyl groups used to modify DNA. Too much formaldehyde could suppress methylation, potentially leading to cancer.