What is Fragile X syndrome?

Chris - This is one of these very interesting classes of diseases where pieces of DNA can change their lengths in an unpredictable way. There's a whole family of this type of diseases and they often affect the brain. This is probably because the brain expresses about 80% of the genes in our body. Therefore, if you have a problem with a particular gene, it will manifest itself in some way in the brain.

With fragile X, we know that one part of the X-chromosome becomes longer than it should do, and it causes genes adjacent to the bits that get too big to behave a bit abnormally.

Whether or not we are in the position to make any differences to those people very soon, I think the answer's probably no.

Mike - Fragile X is called a triplet repeat disease. This is because there are a specific three nucleotides - that's the A, T, G and Cs that Darren was talking about earlier - that repeat themselves a number of times. Everyone has this particular section repeated, but normally it's a limited number of repeats. You may have heard of the idea of the selfish gene. Well there's also selfish DNA. This is bits of DNA that try to replicate themselves and keep going not through the generations but within the cell generations in an organism. And so they make more copies of themselves, partly by unequal crossing over, and it's once you've passed a particular threshold of the number of triplet repeats that you get problems.

The same is true of Huntington's chorea, and there are about four of these diseases.