If you are not achieving desired fragmentation profile after prolonged incubation at 37˚C, to speed up fragmentation, you can incubate the NEBNext UltraShear reaction at 45˚C instead of 37˚C. We do not recommend starting your optimization with 45˚C, since most samples will fragment at 37˚C. Using the higher temperature right away may over fragment the DNA. We recommend testing and optimization of these conditions on your specific sample type and application.