The protein encoded by this gene is a neuron-specific activator of CDK5 kinase. It associates with CDK5 to form an active kinase. This protein and neuron-specific CDK5 activator CDK5R1/p39NCK5A both share limited similarity to cyclins, and thus may define a distinct family of cyclin-dependent kinase activating proteins. [provided by RefSeq, Jul 2008].