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First used in 1990, pre-implantation diagnosis (PID) involves removing a cell from an embryo which has been growing in a petri dish for a few days. The DNA of that cell is tested using ultrasensitive techniques. The test will show whether or not the embryo carries a particular mutation likely to lead to disease. The fertilised egg can then be either implanted in the mother's womb or discarded.

Up to 10 eggs are extracted from women undergoing IVF with the aid of fertility drugs and all the resulting embryos can be screened to see whether they carry the mutation.

Until now PID has been used to detect the sex of an embryo in cases where a family carries a gene which only leads to disease in boys and in gene mutations that invariably cause severe disease in childhood or early adulthood. Extending the procedure to cancers raises trickier issues, say critics, because cancer genes don't inevitably cause disease.