As the name implies, some individuals with extra copies of the MECP2 gene, actually more than one extra copy. This affects a small percentage of cases. While these individuals are commonly identified as having MECP2 Duplication Syndrome, they are sometimes identified as having MECP2 Triplication Syndrome. Both labels are correct. They do, after all, have a duplicate copy of the MECP2 gene, but they also have an additional duplicate.
Symptoms of MECP2 Triplication Syndrome are typically described as similar to MECP2 Duplication, but often more severe than most individuals with duplications but no third copy. Van Esch, writing in Gene Reviews, points out that while the length of duplications does not predict severity, the number of MECP2 genes duplicated does predict severity:
A more important correlation with clinical severity is MECP2 copy number, as triplication of the MECP2 region apparently results in a more severe phenotype.
It is difficult to make exact comparisons between the severity of the two conditions because the number of cases with triplication remains very small and because there is so much variation in severity among those with duplications only. As more cases are identified, research comparing duplications and triplications may be revealing.