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 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:

1. Uridylyltransferases, which transfer uridylyl- groups
2. Adenylyltransferases, which transfer adenylyl- groups
3. Guanylyltransferases, which transfer guanylyl- groups
4. Cytitidylyltransferases, which transfer cytidylyl- groups
5. Thymidylyltransferases, which transfer thymidylyl- groups