An ATP-competitive inhibitor of p38 MAPK isoform p38α (IC50s = 5 nM) and p38β (97% inhibition at 1 µM); has little effect on a range of other kinases, including p38γ and p38δ; blocks the phosphorylation of HSP27, a p38 MAPK substrate, in response to stimulation with anisomycin in HeLa cells (IC50 = 25 nM).