The RecQ DNA helicase WRN was identified as a synthetic lethal target in MSIhigh tumors by several genetic screens. Despite recent advances in the treatment of MSIhigh tumors by immune checkpoint inhibitors, a significant proportion of patients still fails to respond to or relapses after single agent anti-PD1 or combination of anti-PD1 plus anti-CTLA4 treatments. We will present the biochemical, cellular and pharmacological characterization of the first potent and selective WRN helicase inhibitor, HRO761. We will show that HRO761 is an allosteric WRN inhibitor that binds at the interface of the D1 and D2 helicase domains, thus locking WRN in an inactive conformation. We will further show that pharmacological inhibition of WRN by HRO761 recapitulates the phenotype observed by WRN genetic suppression, leading to activation of the DNA damage response and inhibition of tumor cell growth selectively in MSIhigh but not in MSS cells. In addition, we will show that WRN inhibition leads to WRN protein degradation only in MSIhigh tumor cells and give insights into the mechanism of WRN degradation upon DNA damage response induction. A phase 1 clinical trial with HRO761 is currently ongoing to assess the safety, tolerability and preliminary anti-tumor activity in patients with MSIhigh colorectal cancer and other MSIhigh solid tumors