As far back as 1890, it was recognized that there are more men than women in institutions for the mentally handicapped. Studies carried out in the following years confirmed that men outnumber women by 20-30% when it comes to patients with MR. This led to the assumption that genes located on the X-chromosome play a major role in MR.

The first important evidence for this theory came in the 1970s, with the discovery of the Fragile X syndrome. Several other X-linked forms of MR were soon identified. These disorders all belong to the group of X-linked Mental Retardation (XLMR).

	Xhealthy X chromosome
	Xdamaged X chromosome
	Yhealthy Y chromosome
	healthy person
	affected person
Figure 1: pedigree showing X-linked inheritance

Figure 1 shows how an X-linked disorder (XLMR in this case) is inherited within a family. The main characteristic of this type of inheritance is that only men are affected by the disorder. This is because men do not have a second (healthy) copy of the X-chromosome which can compensate for the damaged copy. Instead of a second X-chromosome, like in women, men have a Y chromosome. The Y chromosome can not compensate for a damaged X-chromosome, because their functions are largely dissimilar.

From the above follows that women are seldomly or never affected by an X-linked disorder. They can however be carriers of the disease, in which case the healthy X-chromosome can compensate sufficiently and leave the women in question unaffected. In very rare cases a woman can receive two damaged copies of the X-chromosome and develop the disorder.

Nowadays, XLMR is divided into two groups:

• Non-syndromal or non-specific XLMR, named MRX
• Syndromal XLMR, named MRXS

In MRX, the mental retardation is the only symptom of the disorder, without any neurological, metabolic or dysmorphic features.

The group of patients with MRX are the focus of our research. The identification of new genes involved in MRX has been particularly successfull over the past few years. A total of 16 MRX genes are known today. The close cooperation of several research groups and the establishment of the Euro-MRX Consortium have played a major role in this success.