Lapidot and colleagues found that only the population of leukaemic cells that were positive for CD34 and negative for CD38 (CD34+CD38−) could initiate leukaemic engraftment in immune-deficient mice, and that the frequency of these tumour-initiating cells was about one per million cancer cells (Lapidot et al., 1994; Bonnet and Dick, 1997). Here, CD34 is linked to cancer.