A-3 hydrochloride, a potent, cell-permeable, reversible, ATP-competitive non-selective antagonist of various kinases, againsts cAMP-dependent protein kinase (PKA) (Ki=4.3 µM), casein kinase II (CK2) (Ki=5.1 µM) and myosin light chain kinase (MLCK) (Ki=7.4 µM), also inhibits Protein Kinase C (PKC) and casein kinase I (CK1) with Ki values of 47 µM and 80 µM, respectively.