Collagen is the most abundant protein in the human body and present in connective tissues such as cartilage, bones, tendons, ligaments and skin. Display footnote number: 1 The collagen superfamily comprises 28 members, of which type I collagen is the most abundant form (∼80-85% of total collagen). Also a major protein in the extracellular matrix (ECM) of human cells, collagen I assembles into fibers that form the structural and mechanical scaffold (matrix) of skin and other connective tissues.