Earlier reports from our group showed that mouse and human CD4+ T cells stimulated in vitro with anti-CD3 and the Amaranthus leucocarpus lectin (ALL) activate and proliferate similarly to anti-CD3/CD28 activated cells (15, 16), suggesting that other molecules can generate initial costimulatory signals and, potentially, the existence of an alternate T cell co-activation mechanism. Here, CD4 is linked to acute lymphoblastic leukemia.