Calcium/calmodulin-dependent protein kinase I is expressed in many tissues and is a component of a calmodulin-dependent protein kinase cascade. CAMK1 protein truncated at residue 306 was unable to bind calcium/calmodulin and lacked kinase activity, whereas a CAMK1 protein truncated at residue 294 also did not bind calcium/calmodulin but retained kinase activity even in the absence of calcium/calmodulin. These results were consistent with essential portions of the autoinhibitory and calcium/calmodulin-binding domains encompassing residues 295 to 306 and residues C-terminal to 306, respectively. By mutation analysis, the authors showed that CAMK1 requires thr177 for its phosphorylation. Furthermore, they found that CAMK1 activity is enhanced by CAMK1 kinase in the presence of calcium/calmodulin.