Cyclin dependent kinase 5 (Cdk5) is a key regulator of cell cycle progression in neuronal differentiation that physically associates with and is activated by the neuron-specific protein p35. CDK5RAP1 (Cdk5 regulatory subunit-associated protein 1), also known as Cdk5 activator-binding protein C42, is a 601 amino acid protein that specifically inhibits Cdk5 activation by p35 through formation of a dimer that inhibits kinase activity. CDK5RAP1 contains one TRAM domain, which is thought to bind tRNA and deliver the RNA-modifying enzymatic domain to its target. There are 4 named isoforms of CDK5RAP1 that are produced as a result of alternative splicing events and are expressed at high levels in heart and skeletal muscle.