Fibrinogen (factor I) is a glycoprotein in vertebrates that helps in the formation of blood clots. It consists of a linear array of three nodules held together by a very thin thread which is estimated to have a diameter between 8 and 15 Angstrom (Å). The two end nodules are alike but the center one is slightly smaller. Measurements of shadow lengths indicate that nodule diameters are in the range 50 to 70 Å. The length of the dried molecule is 475 ± 25 Å.