PIIINP, also known as the collagen alpha-1(III) chain, is a protein that in humans is encoded by the COL3A1 gene. Three alpha 1 chains are required to form the type III collagen molecule which has a long triple-helical domain. Type III collagen, an extracellular matrix protein, is synthesized by cells as a pre-procollagen. It is found as a major structural component in hollow organs such as large blood vessels, uterus and bowel. Other functions of type III collagen include interaction with platelets in the blood clotting cascade and it is also an important signalling molecule in would healing. Mutations in the COL3A1 gene cause the vascular type of Ehlers-Danlos syndrome.