Entospletinib (GS-9973) is an orally bioavailable, selective Syk inhibitor with IC50 of 7.7 nM in a cell-free assay and showed 13- to >1000-fold cellular selectivity for Syk over other kinases(including Jak2, ckit, Flt3, Ret, KDR) as assessed by target protein phosphorylation or functional response.