A cell-permeable, 4-thiazolyl-2-phenylaminopyrimidine compound that acts as a potent, reversible, and ATP-competitive inhibitor of spleen tyrosine kinase (Syk) (Ki=9nM). Exhibits excellent selectivity over Cdk2 (Ki=3.33uM), ROCK (Ki=2.3uM), Src (Ki=3.33uM), and ZAP70 (Ki=240nM). Shown to inhibit mast cell degranulation (IC50=70nM) that is commonly observed in asthma and other allergic diseases.