Kinase
An enzyme that switches other proteins on by sticking a phosphate group onto them. Cancer signalling runs on kinases, and kinase inhibitors are the biggest class of targeted drugs.
The human genome encodes about 500 kinases; many sit in growth pathways, including the inner part of receptors like EGFR and HER2 and the relay proteins BRAF, MEK, ALK, CDK4/6 and PI3K. A mutated kinase that stays permanently active is one of the commonest cancer drivers, and because kinases share a pocket that binds ATP (the phosphate donor), small molecules designed to occupy that pocket can shut them off. Imatinib against BCR-ABL in 2001 proved the idea; dozens of kinase inhibitors now exist, and successive generations are engineered to overcome the resistance mutations that arise in the pocket.