Nucleoside triphosphates are crucial mediators of life. The high energy phosphoanhydride bond of (usually) ATP is used to drive unfavorable chemical reactions, to fuel biological machines, and to regulate a vast number of processes via phosphorylation of proteins. GTP, in turn, is used almost exclusively for the regulation of signal transduction and transport processes, whereas the other nucleotides play a less important role, except in synthesis pathways involving sugars (UTP) and phospholipids (CTP) and as building blocks of polynucleotides such as RNA and DNA. Proteins that bind and use these nucleotides for enzymatic reaction and regulation are very diverse. Although some of them constitute the largest protein superfamilies known (e.g. protein kinases), others seem to be far less conserved in evolution.