tumour cells that are capable of repopulating the tumour. Parada points out:“this demonstrates a tumour cell hierarchy in which a relatively quiescent CSC (that serendipitously expresses our transgene) gives rise to actively dividing cells. Better understanding of the CSCs will be critical for re-evaluation of existing therapies and development of new ones.” The final study, led by Hans Clevers, also used lineage tracing to demonstrate the presence of CSCs, but in mouse models of primary intestinal adenomas. Taken together, these three studies add considerable weight to the theory of CSCs and may drive future clinical research.