Notch 1, for example, has 36 of these repeats. Each EGF-like repeat is composed of approximately 40 amino acids, and its structure is defined largely by six conserved cysteine residues that form three conserved disulfide bonds. Each EGF-like repeat can be modified by "O"-linked glycans at specific sites. An "O"-glucose sugar may be added between the first and second conserved cysteines, and an "O"-fucose may be added between the second and third conserved cysteines. These sugars are added by an as-yet-unidentified "O"-glucosyltransferase (except for Rumi), and GDP-fucose Protein "O"-fucosyltransferase 1 (POFUT1), respectively. The addition of "O"-fucose by POFUT1 is absolutely necessary for notch function, and, without the enzyme to add "O"-fucose, all notch proteins fail to function properly. As yet, the manner by which the glycosylation of notch affects function is not completely understood.