To evaluate whether ongoing recombinase activity could occur in established ALK+ ALCL tumours, we looked for RAG1 transcripts in 52 cases, but all were negative, as were all the three ALCL cell lines tested in contrast to the immature T-ALL cell lines arrested at a thymic stage of development (Supplementary Fig. 3B). This evidence concerns the gene RAG1 and acute lymphoblastic leukemia.