Cell-permeable. A potent bisindolylmaleimide inhibitor of protein kinase C (PKC) isoforms (IC₅₀ = 5, 24, 14, 27, and 24 nM for PKC-α, PKC-βI, PKC-βII, PKC-γ, and PKC-ε, respectively). It inhibits protein kinase A (PKA) (IC₅₀ = 0.9 μM) and calcium/calmodulin kinase II (IC₅₀ = 17 μM) at much higher concentrations. Also inhibits glycogen synthase kinase-3 (GSK-3) (IC₅₀ = 6.8 nM).