The phosphorylation and dephosphorylation of proteins on serine and threonine residues is an essential means of regulating a broad range of cellular functions. The protein phosphatase (PP) holoenzyme is a trimeric complex composed of a regulatory subunit, a variable subunit, and a catalytic subunit. PR48 is a 48 kDa regulatory subunit of protein phosphatase 2A (PP2A). PP2A activity is required for the initiation of DNA replication in yeast, viral, and vertebrate systems. PR48 localizes to the nucleus and binds specifically to Cdc6, a highly conserved protein which is required for the formation of prereplicative complexes. PR48 is considered to be involved in the dephosphorylation of Cdc6 by PP2A, a process important to the control of DNA replication in mammalian cell.