Hydroxylysine (Hyl) is an amino acid with the molecular formula C6H14N2O3.

It was first discovered in 1921 by Donald Van Slyke as the 5-hydroxylysine form.

[2] It is biosynthesized from lysine via oxidation by lysyl hydroxylase enzymes.

However, the enzyme JMJD6 has recently been shown to be a lysyl hydroxylase which modifies an RNA splicing factor producing the (5S) stereoisomer.

Additionally, in E. coli, there has been at least one lysine N-hydroxylase enzyme identified, named IucD.