Primary or secondary amino groups are ubiquitous in biomolecules. They are present in proteins and peptides as ε-amino groups of lysines and N-terminal amino groups. Many natural and synthetic small molecules are amines, and DNAs with amino groups can be prepared by solid phase synthesis and enzymatically.

Amino groups can be modified by acylation with various activated carbonyl compounds. The most common for these purposes are N-hydroxysuccinimide esters (NHS esters) and sulfotetrafluorophenyl ethers (STP ethers).

STP ethers are reactive derivatives that react efficiently with primary amines to form the same acylation products as NHS esters. They are dramatically more water-soluble than the corresponding NHS esters, making them ideal for labeling biomolecules in an aqueous environment.