CHEK1 and neoplasm: The susceptibility of ecDNA-containing tumour cells to CHK1 inhibition was confirmed with three structurally different CHK1 inhibitors—GDC-0575, SRA737 and CHIR-124—whereas the checkpoint kinase 2 (CHK2) inhibitor CCT241533 showed no differential inhibitory effect between ecDNA+ and ecDNA− isogenic cell lines (Extended Data Fig. 8a–e).