There are clinical implications of this theoretical model. The different duration of maintenance required for boys and girls may reflect differences in time taken to re-establish the immune system, which in turn may reflect differences in social behavior. It may be possible, by suitable immune conditioning, to reduce the required duration of maintenance therapy for both boys and girls; this might be achieved by, for example, a planned immunization program and/or by encouraging contacts outside the immediate family. A sound theoretical basis for maintenance therapy would assist in the rational design of trials to refine this currently least well-understood aspect of treatment of childhood ALL.