nic ectoderm (teeth, hair, nails, glands). The individuals observed by Darwin were exclusively males and their symptoms resulted from a mutated gene, called ED1, recently identified on the X-chromosome. Females rarely get this form of ectodermal dysplasia because, unlike males who have one X and one Y chromosome, females have two X's and therefore carry a backup gene that can compensate for the mutation. However, there are families where children of both sexes are affected, with no evidence of X-chromosome involvement.

It was readily apparent to scientists like Zonana and Monreal that other gene defects could also produce hypohidrotic ectodermal dysplasia. Their group was able to identify the chromosome 2 gene, named DL, with the help of "downless" mice. These animals derive their name from a sparseness of hair as well as other features that mimic the human disorder. The researchers used DNA sequence information for the mouse gene to find the equivalent human gene on chromosome 2.

Once the gene was located and sequenced, the investigators examined families that had affected male and female members, yet no linkage with the X-chromosome. Five families had members with abnormalities in the DL gene, with each family having a unique mutation. The gene could be dominant (a defective gene from one parent was sufficient to cause the disorder) or recessive (both parents had to contribute a mutated gene.)

A possible explanation for this observation, according to Zonana, lies in the part of the gene affected by the mutation and in the predicted structure of the gene's protein product. The gene sequence is characteristic of a class of proteins that span the cell membrane and act as chemical receptors. Each receptor molecule binds with a signaling molecule and, as a result, causes a message to be sent to the cell nucleus. The part of the receptor molecule outside of the cell membrane acts "one on one" to bind a complementary signaling molecule. However, the
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Contact: Wayne Little
wayne.little@nih.gov
301-594-7560
NIH/National Institute of Dental and Craniofacial Research
3-Aug-1999

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