ZAP-70 enhances BCR signaling in B-CLL cells by promoting CD79 phosphorylation. In the proposed model, the increased levels of CD79a and CD79b phosphorylation in B-CLL cells expressing ZAP-70 are dependent on the presence of the 2 ZAP-70 SH2 domains. The mechanisms via which ZAP-70 enhances phosphorylation of CD79a/CD79b are not yet known, but may be the result of a direct interaction between the ZAP-70 SH2 domains and the phosphorylated ITAMs. The phosphorylation of CD79a/CD79b would result in an augmented phosphorylation of Syk and activation of downstream signaling cascades while limiting BCR internalization. In the absence of ZAP-70, the decreased phosphorylation of CD79a/CD79b would limit Syk phosphorylation and downstream signaling but would result in significantly higher levels of BCR internalization.