Approximately 20% of children with ALL will experience a relapse of their disease following treatment. Of these, most will never be cured.

"We have previously shown that these relapses were due to small numbers of cells which survived the treatment administered to the patient," said Prof Norris.

"However, it has been unclear whether these cells developed resistance to chemotherapy during the course of treatment or if they were already present in the child at the time their cancer was diagnosed."

The findings of this study demonstrate that relapse in ALL patients can result from a minor, but intrinsically resistant subpopulation of cells, present from the time of diagnosis. Undetected at diagnosis because of their very small numbers, this population of leukaemia cells remained in the patients body throughout the disease and continued to thrive even after the major population of sensitive leukaemia cells were destroyed and the patient appeared to have gone into remission.

"We have shown, for the first time, that children with a greater number of these cells at diagnosis are more likely to experience relapse much sooner after treatment.

"Without new strategies to identify and attack these cells early in treatment, relapse appears inevitable for these patients," said Prof Norris.

"Our ability to characterise these cells means that researchers can now develop treatments which specifically target the cells and are more effective than current options."