BCR and acute lymphoblastic leukemia: These results together with other studies have led to a model in which BCP-ALL are arrested at the pre-BCR checkpoint due to one of the following mechanisms [14, 15]: (i) expressing a pre-BCR that mediates signals supporting survival and proliferation but inhibiting differentiation, (ii) bypassing the pre-BCR checkpoint, or (iii) blocking/down-regulating pre-BCR expression, and through alternative mechanisms activating proliferation and survival.