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Keratoconus in One Identical Twin

A 23-year-old woman had bilateral keratoconus requiring keratoplasty, and her identical twin sister had normal corneas. Red blood cell antigen and biochemical marker typing supported monozygosity. Differences between the twins in eye-rubbing and hormonal status may have contributed to the development of keratoconus in one and not the other. Since the second twin may yet develop keratoconus, these findings do not negate the importance of heredity in the disease; they do suggest, however, that nongenetic factors can influence the manifestation of keratoconus in susceptible individuals.