Epidermal growth factor: Internal duplication and probable relationship to pancreatic secretory trypsin inhibitor

The sequence of the first half of epidermal growth factor is related to that of the second half, indicating a gene elongation through duplication; it is also similar to that of pancreatic secretory trypsin inhibitor, which suggests a common evolutionary origin for these two proteins. Statistical evaluation shows that it is very unlikely that either similarity in sequence could have occurred by chance. The two proteins may also be functionally related, as each forms a complex with an arginine esterase.