Nucleotidyltransferases are transferase enzymes of phosphorus-containing groups, e.g., substituents of nucleotidylic acids or simply nucleoside monophosphates. The general reaction of transferring a nucleoside monophosphate moiety from A to B, can be written as: A-P-N + B ⇌ {\displaystyle \rightleftharpoons } A + B-P-N For example, in the case of polymerases, A is pyrophosphate and B is the nascent polynucleotide. They are classified under EC number 2.7.7 and they can be categorised into: Uridylyltransferases, which transfer uridylyl- groups Adenylyltransferases, which transfer adenylyl- groups Guanylyltransferases, which transfer guanylyl- groups Cytitidylyltransferases, which transfer cytidylyl- groups Thymidylyltransferases, which transfer thymidylyl- groups Source: Wikipedia (en)