Rett syndrome is a severe neurodevelopmental condition, currently diagnosed by application of recently revised clinical criteria. Since the discovery in 1999 of the association between MECP2 mutations and Rett syndrome, there has been considerable research into the relationship between genotype and phenotype in this disorder. This research has been able to link a number of the more common mutations to milder or more severe clinical features or to specific Rett syndrome variants.[3, 4, 5] In addition to the common point mutations (p.R106W, p.R133C, p.R168X, p.R255X, p.R270X, p.R294X, p.R306C and p.T158M) of MECP2 which are present in between 60% to 70% of cases with pathogenic MECP2 mutations,[3, 4, 6, 7] deletions in the C-terminal region have been reported in approximately 6-14% of cases.[3, 4, 7, 8] In one study the phenotype of ten cases with C-terminal deletions has been examined in detail, but in no other study would they appear to have been specifically examined as a separate group. Using the data collected as part of InterRett, an international Rett syndrome phenotype database, and including data from the Australian Rett Syndrome Database, a population-based cohort, the phenotype of C-terminal deletions is described and compared with the phenotype of cases with all other mutations and also with recognised severe and mild mutations.