Mobilization: one sc injection of 200 µg/kg pegfilgrastim (Neulasta, Amgen) Evaluation during the study: CD34 circulating cells from day 3 to day 7 ; AE recording Judgment criterion: percentage of children achieving at least 5x10e6 CD34 cells with a standard apheresis (less than 3 blood volume processed) Analysis: sequential Bayesian study