CD4 and neoplasm: Before tumor onset, these ALK+ cells were either CD4+ or CD8+ T cells (i.e., TCRβ+), whereas in tumor-bearing mice, additional CD4−CD8−TCRβ− tumor cells were detectable, suggesting that a fraction of ALK-transformed DN tumor cells was able to leave the thymus, or that the TCR expression is silenced after exit from the thymus as suggested previously (45) (Fig S2B).