"Jumping genes" or transposable elements are long stretches of repetitive DNA that can insert themselves throughout the genome. Normally, they are held inactive by a series of marks along the genome. But when these marks are erased, activated transposons can disrupt critical genes, causing dramatic defects in development. Here are two flowers: on the left is a normal plant where transposons are silent; on the right, transposons have been activated, severely mutating the flower. Researchers at CSHL have discovered a new fail-safe mechanism that prevents transposon activation even when the inactivating marks have been erased.