Three types of unusual post-translational modification have been found within conserved amino acid sequences in epidermal growth factor homology regions (EGF modules) of some multidomain proteins. β-Hydroxyaspartate and β-hydroxyasparagine are found within -Cys-Xxx-Asp/Asn-Xxx-Xxx-Xxx-Xxx-Tyr/Phe-Xxx-Cys-Xxx-Cys- sequences. (Xylα1→3)Xylα1→3Glcβ1→O-Ser glycans at conserved sites within -Cys-Xxx-Ser-Xxx-Pro-Cys- sequences have been reported in several proteins. Fucα1→O-Thr/Ser Modifications have been found at conserved sites within -Cys-Xxx-Xxx-Gly-Gly-Thr/Ser-Cys- sequences. More recently, it has been discovered that the Ser residue corresponding to the potential O-fucosylation site in human factor IX carries the novel tetrasaccharide NeuAcα2→6Galβ1→4GlcNAcβ1→3Fucα1→O-Ser; this tetrasaccharide can be considered to be an extension of the Fucα1→O moiety. The consensus sequences for these post-translational modifications are in close proximity to each other; e.g. human factor IX has all three unusual modifications within a 12 amino acid linear sequence. In proteins with multiple EGF modules, the O-glycosidic modifications have been found only within the N-terminal EGF module; β-hydroxyaspartate/asparagine residues are not restricted in the same fashion. Little is known yet about the functions of, or possible relationships between, any of these modifications.