Bilateral macrostomia in one of monozygotic twins James K. Hartsjeld, Jr., D.M.D., MS.,* and David Bixler, D.D.S., Ph.D., Indianapolis, Ind. DEPARTMENT OF ORAL FAClAL GENETICS, INDIANA UNIVERSITY SCHOOL OF DENTISTRY 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. (ORAL SURG. 52648-651, 1984) L ateral facial clefting is a rare phenomenon, which may be isolated or can occur in association with other anomalies. Fogh-Andersen’ reported an inci- dence in Denmark of about one lateral facial cleft in 330 cases (0.3%) of typical lip and palate clefts. Since the frequency of cleft lip with or without cleft palate is about one in 750 births in Denmark,2 the overall incidence of macrostomia is estimated at 1 per 225,000 births, a rare event indeed. Such macro- stomia occurs, either unilaterally or bilaterally, in about 15% of all patients with Treacher Collins syndrome,3and it also occurs in at least one third of all patients with mandibular ramus agenesis in the facie-auriculo-vertebral spectrum or hemifacial microsomia.3 Macrostomia is typically mild, unilat- eral, more common on the left side, and more common in males.3 However, it is clear that nonsyn- dromic, symmetrical bilateral macrostomia is a very rare event, with only four such reports known to us.“-’ The present concerns a pair of monozygotic twins, one of which (twin A) had bilateral symmetric clefting of the mouth. This rare developmental mal- formation was of particular interest not only because it occurred in only one of a pair of identical twins but also because the specific anatomy of the cleft permit- ted speculation about formation of the mouth in the embryo. Fig. 1. External oral view of twin A illustrating notches in the vermilion border medial to the commissures. CASE REPORT These female twins were the product of a 36- to 37week gestation, with fetal activity beginning at ahout 18 weeks. Supported in part by the Oral Facial Genetics Training Grant DE 7043 from the National Institutes of Health. *At present Research Fellow in Orthodontics at Harvard School of Dental Medicine, Boston, Mass. Fig. 2. Mouth of twin B showing no abnormalities. 048