Primary amino targets for the action of formaldehyde. In addition to the primary amino group of the lysine residue the terminal amino group is also a potential target depending on the surrounding environment. Lysine is shown here (top) as the N-terminal amino acid; the terminal amino group and polypeptide backbone are illustrated in blue. The primary amino groups are highlighted in yellow. Three of the four DNA bases (bottom) possess primary amino groups, which are essential for H-bonding in the double helix structure of DNA. Any modification of the amino groups of the bases by formaldehyde may occur only in single stranded regions. Adapted from Hart (1987).