Regulatory subunit of the cAMP-dependent protein kinases involved in cAMP signaling in cells. The inactive holoenzyme is composed of two regulatory chains and two catalytic chains. Activation by cAMP releases the two active catalytic monomers and the regulatory dimer. Interacts with PRKX; regulates this cAMP-dependent protein kinase. Four types of regulatory chains are found: I- alpha, I-beta, II-alpha, and II-beta. Their expression varies among tissues and is in some cases constitutive and in others inducible. Belongs to the cAMP-dependent kinase regulatory chain family. Note: This description may include information from UniProtKB.