Casein kinase II (CKII) is a serine/threonine protein kinase that is required for progression through the cell cycle but whose major functions are not clearly understood. CKII is composed of a heterotetrameric complex that includes alpha and alpha-prime catalytic subunits and a beta non-catalytic subunit. Although from independent genes, CKII alpha and CKII alpha-prime can compensate for each other for some functions. However in response to ionizing radiation (IR) damage, CKII alpha and CKII alpha-prime appear to function independently in inhibiting apoptosis signaling.