Kinase function and dysregulation. Protein kinases constitute one of the largest protein families, with roughly 518 kinases encoded in the human genome. Kinases share a catalytic domain for adenosine triphosphate (ATP) binding, catalyzing the transfer of its γ-phosphate group to serine, threonine, or tyrosine residues of themselves (autophosphorylation) or of other proteins. Protein phosphorylation is the major mechanism through which protein function is regulated and is fundamental to most aspects of cell life. A variety of diseases including cancer, inflammation, and autoimmune disorders, are associated with aberrant regulation of protein kinases. Dysregulation can occur due to mutations, overexpression, chromosomal rearrangements, or gene amplification. Thus over the past 20 years, protein kinases have become one of the most important classes of drug targets, especially in the field of oncology. Kinase groups. Protein kinases are generally divided into eukaryotic protein kinases, which share a similar sequence and structure, and atypical protein kinases, which have biochemical kinase activity, but lack sequence similarity to the typical kinase domain. Furthermore, eukaryotic protein kinases can be classified based on their sequence identity into 8 main kinase groups: AGC, CAMK, CK1, CMGC, STE, TK, TKL, and Other. Kinase structure. Protein kinase structures consist of two domains, i.e. the N-and C-lobes, connected via a hinge region. The majority of kinase inhibitors target the catalytic cleft between these lobes, which contains the highly conserved ATP binding site. Based on over 1,200 kinase-ligand crystal structures, van Linden et al. have defined the binding site to comprise 85 residues and 19 well-defined regions/motifs, covering a (i) front cleft, (ii) gate area, and (iii) back cleft (important regions shown in Figure .B and their KLIFS numbering in brackets in the following): (i) ATP solely occupies the front cleft, which contains the hinge region (46-48), linker (49-52), glycine-rich loop (4-9), and catalytic loop (68-75). ATP's adenine group as well as most kinase inhibitors form hydrogen bonds to the hinge region. (ii) The gate area contains the conserved DFG motif (81-83), the conserved lysine residue K17 (17), and the gatekeeper residue (45), often used for inhibitor selectivity, preceding the hinge region. (iii) The back cleft contains amongst others the αC-helix (20-30), including the conserved glutamine residue E24 (24), which forms the conserved K17-E24 salt bridge in the so-called αC-in (as opposed to the αC-out) conformations. Furthermore, the DFG motif can undergo a significant conformational change, which results in an inactive state of the kinase (DFG-out instead of DFG-in conformation). This DFG-flip opens a hydrophobic region in the back cleft targeted by inhibitors stabilizing the inactive state. The KLIFS database has made this and further information about the available kinases structures, their bound ligands, and the interactions between them freely available.