CHEK1 and neoplasm: We next inhibited CHK1 pharmacologically using CHIR-124 (ref. 25) and found that ecDNA-containing tumour cells were more sensitive to CHK1i than their corresponding isogenic HSR cells, with a half-maximal inhibitory concentration (IC50) approximately 4-fold higher in COLO320HSR compared to COLO320DM cells (Fig. 3d).