Clinical findings in individuals with duplication of genes associated with X‐linked intellectual disability

Duplication of all genes associated with X-linked intellectual disability (XLID) have been reported but the majority of the duplications include more than one XLID gene. It is exceptional for whole XLID gene duplications to cause the same phenotype as sequence variants or deletions of the same gene. Duplication of PLP1, the gene associated with Pelizaeus-Merzbacher syndrome, is the most notable duplication of this type. More commonly, duplication of XLID genes results in very different phenotypes than sequence alterations or deletions. Duplication of MECP2 is widely recognized as a duplication of this type, but a number of others exist. The phenotypes associated with gene duplications are often milder than those caused by deletions and sequence variants. Among some duplications that are clinically significant, marked skewing of X-inactivation in female carriers has been observed. This report describes the phenotypic consequences of duplication of 22 individual XLID genes, of which 10 are described for the first time.