Allysine is a derivative of lysine that features a formyl group in place of the terminal amine.  The free amino acid does not exist, but the allysine residue does.  It is produced by aerobic oxidation of lysine residues by the enzyme lysyl oxidase.  The transformation is an example of a post-translational modification.  The semialdehyde form exists in equilibrium with a cyclic derivative. 
Allysine is involved in the production of elastin and collagen.  Increased allysine concentration in tissues has been correlated to the presence of fibrosis.
Allysine residues react with sodium 2-naphthol-6-sulfonate to produce a fluorescent bis-naphtol-allysine product.  In another assay, allysine-containing proteins are reduced with sodium borohydride to give a peptide containing the 6-hydroxynorleucine (6-hydroxy-2-aminocaproic acid) residue, which (unlike allysine) is stable to proteolysis.