To determine whether the lymphoid cell that is a target for leukemic transformation is a rare cell that has undergone a functional Igh or Tcrb gene rearrangement despite the lack of Rag1 protein, as is the case with scid mice that develop pre-T-ALL[32], we searched for evidence of clonal Igh or Tcrb gene rearrangements in the B and T cell leukemias, respectively. The gene discussed is RAG1; the disease is acute lymphoblastic leukemia.