DNA-directed RNA polymerases, classified as EC 2.7.7.6 and generally called RNA polymerases, are the enzymes that assemble RNA chains from ATP, GTP, CTP and UTP using a DNA or RNA template. The family includes the polymerases responsible for transcribing genes into messenger RNA in all cellular life as well as the primases that synthesize the short RNA primers required to initiate DNA replication. Their template dependence and nucleotide selectivity make transcription remarkably faithful. Inhibitors of these enzymes, studied mainly in medicine, illustrate how central transcription is to life.