FEN1 Threading (IMAGE) DOE/Lawrence Berkeley National Laboratory Caption The schematic at top illustrates how inversion of the DNA flap keeps the phosphodiester bonds away from the metal catalysts that can inadvertently cut the strand. The bottom view shows a single-stranded DNA flap passing through a small opening in the FEN1 protein, guided by electrostatic forces in the basic region. Credit Susan Tsutakawa/Berkeley Lab Usage Restrictions Editorial use only. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.