Changes to DNA On-Off Switches (image) University of Pennsylvania School of Medicine Caption This is a model showing that TIP60/NuA4-dependent acetylation limits 53BP1 binding to histone H4 when present on the same histone H4 tail with the mark H4K20Me2, a recognition mark for 53BP1. A TIP60 deficiency would result in reduced H4 acetylation, and increased 53BP1 binding, thus blocking BRCA1 function at DNA double-strand breaks. Credit Roger A. Greenberg, M.D., Ph.D., Perelman School of Medicine, University of Pennsylvania Usage Restrictions None 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.