Sir

In an excellent News and Views Feature on the future of somatic nuclear-transfer cloning, J. B. Gurdon and Alan Colman (Nature 402, 743; 1999) express concern about the inactive X chromosome when female cells are used in cloning. If the inactive X chromosome does not become reactivated by exposure to egg cytoplasm, they wonder whether embryos reconstructed from a donor containing an inactive maternal X chromosome be viable.

On this point they need have few worries. In early embryos of eutherian mammals, the X chromosome derived from the father is indeed distinguished from that coming from the mother, and is preferentially inactivated in the first two tissues to differentiate (trophoblast and primary endoderm, both of which contribute exclusively to the placenta and extra-embryonic membranes). Thereafter, however, the X chromosomes ‘forget’ which parent they have come from: they are inactivated at random in the cells contributing to the fetus and retain no trace of their parental origin.

Incidentally, it is legal in the United Kingdom to use human eggs to create an embryo, if the aim is to satisfy one of the purposes specified in the 1990 Human Fertilization and Embryology Act (which does not include research on cell or tissue therapy). An amendment seeking to prohibit the use of human eggs to create embryos for research was put forward during the passage of the Act, but was soundly defeated in both Houses of Parliament.