The phosphorylation and dephosphorylation of proteins on serine and threonine residues is an essential means of regulating a broad range of cellular functions in eukaryotes, including cell division, homeostasis and apoptosis. A group of proteins that are intimately involved in this process are the serine/threonine (Ser/Thr) protein kinases. SUDD, also known as RIOK3 (RIO kinase 3), is a 519 amino acid protein that contains one protein kinase domain and belongs to the Ser/Thr protein kinase family. Expressed in a variety of tissues, SUDD catayzes the ATP-dependent phosphorylation of target proteins, thereby influencing signaling events throughout the cell. SUDD is expressed as two isoforms due to alternative splicing events.