CHEK1 and neoplasm: These observations have important ramifications for the clinical development of Chk1 inhibitors as monotherapy agents, namely: (i) tumours with a large fraction of actively replicating cells are predicted to be responsive, (ii) schedules and/or inhibitors that maintain Chk1 inhibition over several days (thereby targeting tumour cells as they enter and progress through replication) may provide greater efficacy, and (iii) quiescent or poorly vascularised regions of a tumour will be refractory to therapy.