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. It arises from a post-translational hydroxy modification of lysine. It is most widely known as a component of collagen.

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. It arises from a post-translational hydroxy modification of lysine. It is most widely known as a component of collagen.