fibrillar collagen trimer collagen type I trimer GO:0005584 A collagen trimer containing alpha(I) chains. The most common form of type I collagen is a heterotrimer containing two alpha1(I) chains and one alpha2(I) chain; homotrimers containing three alpha1(I) chains are also found. Type I collagen triple helices associate to form banded fibrils. Wikipedia:Collagen_type_I