A sequence of about thirty to forty amino-acid residues long found in the sequence of epidermalgrowth factor (EGF) has been shown [<cite idref="PUB00004321"/>, <cite idref="PUB00004609"/>, <cite idref="PUB00015295"/>, <cite idref="PUB00003983"/>, <cite idref="PUB00001555"/>, <cite idref="PUB00015296"/>]to be present, in a more or less conserved form, in a large number of other, mostly animal proteins.The functional significance of EGF domains in what appear to be unrelated proteins is not yet clear.However, a common feature is that these repeats are found in the extracellular domain of membrane-bound proteins or in proteins known to be secreted (exception: prostaglandin G/H synthase). TheEGF domain includes six cysteine residues which have been shown (in EGF) to be involved indisulphide bonds. The main structure is a two-stranded beta-sheet followed by a loop to a C-terminalshort two-stranded sheet. Subdomains between the conserved cysteines vary in length.