“…To address these issues, we have developed a sensitive and fast assay to quantify in vitro E2/E3 enzyme activity using MALDI-TOF MS. It builds on our DUB MALDI-TOF assay ( Ritorto et al., 2014 ), which has enabled us to screen successfully for a number of selective DUB inhibitors ( Kategaya et al., 2017 , Magiera et al., 2017 , Weisberg et al., 2017 ), and adds to the increasing number of drug discovery assays utilizing label-free high-throughput MALDI-TOF MS. Apart from E2/E3 enzymes and DUBs ( Ritorto et al., 2014 ), high-throughput MALDI-TOF MS has now successfully been used for drug discovery screening of protein kinases ( Heap et al., 2017b ), protein phosphatases ( Winter et al., 2018 ), histone demethylases, and acetylcholinesterases ( Haslam et al., 2016 ), as well as histone lysine methyltransferases ( Guitot et al., 2014 ).…”