TOXIC ISOFORM (PRION). “Now: don’t change the gene that codes for PrP. Don’t even change the sequence of amino acids that make up the protein. Just take that perfectly normal PrP protein and tweak the 3-D structure a bit — uncoil some of the helices and re-fold them into flat sheets. Chemically and genetically, this new PrP is identical to the old one. It’s simply shaped differently, like a person who has changed from a sitting to a standing position. It’s a new “isoform”: different in physical appearance but not in genetic or chemical makeup. Don’t eat this one.”
http://www.strangehorizons.com/2004/20040216/killer.shtml