* Proline (Pro or P abbreviation) is an alpha-amino acid. It is one of the proteinogenic amino acid and DNA encoded by the CCU, CCC codons, CCA, and CCG. The human body can synthesize proline from other compounds, therefore it is not a dietary essential amino acid. It is the only amino protein acid which incorporates in its molecule a secondary amine, namely the amine nitrogen which is bonded to two alkyl groups (R1-N (H) -R2).

* Hydroxyproline is an amino acid found in collagen. The excretion of hydroxyproline in urine is a useful indicator of metabolism (degradation) of collagen. It reflects the rate of degradation of the bone, so it is a good test for the tracking of the situation in Paget's disease.