BCR and B-cell chronic lymphocytic leukemia: Many of the in vitro effects of fostamatinib have been recapitulated in vivo using the Eμ-TCL1 transgenic mouse model of CLL.71 In this model, fostamatinib treatment results in inhibition of BCR signaling, an initial transient lymphocytosis, reduced proliferation and survival of the malignant B cells, and prolonged survival of the treated animals.