Kinases are enzymes that catalyze the transfer of a phosphate group from ATP to a substrate. They play critical roles in metabolism and particularly in cell signaling and cell cycle regulation. Kinases have been widely pursued as drug targets, and continue to be of high interest within the drug discovery community. As of the end of 2023, 87 kinase inhibitors have been approved as drugs by the United States Food and Drug Administration (FDA), with many more in clinical development. Most kinase inhibitors bind at the nucleotide binding site and function as competitive inhibitors with respect to ATP. Inhibitors that are ATP site binders have been categorized into several classes depending on whether they bind to the active form of the enzyme where the Asp residue in the conserved "DFG loop" is pointed in towards the active site (Type I), or to an inactive, "DFG-out" conformation (Type II). A minority of kinase inhibitors bind at other sites that lead to orthosteric (Type III) or allosteric (Type IV) modulation of catalytic function. Kinases are generally considered to be highly druggable, at least with respect to their ATP binding site. However, in several reported kinase inhibitor programs the compound that was eventually advanced into the clinic was a MC.