Should embryos with a hereditary disorder be transferred if no unaffected embryos are available?

The numbers of cycles of preimplantation genetic diagnosis or screening are rising steadily in Europe with over 2,700 reported in 2004 (the most recent year for which data are available). Fertility centres are able to screen for a growing number of genetically related conditions, but what should doctors do if no embryos without the targeted condition are available for transfer and the parents request that affected embryos should be transferred instead?