Bilateral macrostomia in one of monozygotic twins.

This report concerns a pair of monozygotic twins discordant for bilateral symmetrical macrostomia. Laterally placed, symmetrical notches in the vermilion borders of both upper and lower lips were apparent in the affected twin. Since the oral cavity walls develop by a differential growth and merging mechanism, this result suggests a mechanism of tissue breakdown as the cause of macrostomia. Furthermore, discordance in these monozygotic twins minimizes the importance of heredity in this malformation.