Spleen tyrosine kinase (Syk) is a non-receptor tyrosine kinase that, upon phosphorylation, binds to immunoreceptor tyrosine-based activation motifs of FcRγ chains and mediates downstream signaling related to platelet function and inflammation. Syk inhibitor II is a cell-permeable, pyrimidine-carboxamide compound that selectively and reversibly blocks Syk (IC50 = 41 nM) in an ATP-competitive manner. It is much less potent against PKCε, PKCβII, ZAP-70, Btk, and Itk (IC50s = 5.1, 11, 11.2, 15.5, and 22.6 μM, respectively).