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. The salt-inducible kinases (SIKs) are a family of related serine-threonine kinases and are key enzymes that modulate important processes such as steroid hormone biosynthesis and insulin signaling in adipocytes. QSK, also known as L19 or SIK3 (salt-inducible kinase 3), is a 1,263 amino acid cytoplasmic protein belonging to the protein kinase superfamily, CAMK Ser/Thr protein kinase family and the AMPK subfamily. Ubiquitously expressed, QSK consists of one protein kinase domain and a UBA domain. QSK is activated by 14-3-3 Ω and utilizes magnesium as a cofactor. QSK exists as three alternatively spliced isoforms.