the cross linking of dna by formaldehyde causes genes to be silenced. this is fine and dandy if it's in a junk bit of dna or in a specialised cell that doesn't need that gene. but if it interacts with p53 or other oncogenes things get nasty.   it's not always synthetic. your body naturally makes and metabolises some all the time. when you're exposed to larger amounts like being an embalmer etc that's when things go wrong.
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