Twenty percent children and 65% of adults relapse following initial cancer therapy.New drugs developed for treatment of such patients are warranted.Velcade, a newly introduced proteasome inhibitor meets this demand and thus, has been proposed to treat refractory childhood leukaemia.However, whether it exerts deleterious effects on the gonad is unknown.The aim of the present study was to evaluate the side effects of one cycle ofvelcade treatment on the testes of young males in a mouse model.Mechanisms of the velcade-induced damage were evaluated.35 dold male mice were injected i.p with velcade (lmg/kg) at an interval of day 1 (35 days), 4, 8 and 11 respectively.Animals were sacrificed either 2 or 45 days after last injection and referred to as short-and long-term follow-up.