Monozygotic twin brothers with the fragile X syndrome: different CGG repeats and different mental capacities

Little is known about the mechanism of CGG instability and the time frame o f instability early in embryonic development in the fragile X syndrome. Dis cordant monozygotic twin brothers with the fragile X syndrome could give us insight into the time frame of the instability. We describe monochorionic diamniotic twin brothers with the fragile X syndr ome who had different CGG repeats and different mental capacities, whereas the normal mother had a premutation. The more retarded brother had a full m utation in all his cells and no FMR-1 protein expression in lymphocytes, wh ereas the less retarded brother had 50%/50% mosaicism for a premutation and full mutation and FMR-1 protein expression in 26% of his lymphocytes. The differences in repeat size could have arisen either before or after the time of splitting. The time of splitting in this type of twin is around da y 6-7. Given the high percentage of mosaicism, we hypothesise that the inst ability started before the time of splitting at day 6-7.