When Rett syndrome is due to genes other than MECP2 .

Two individuals meeting diagnostic criteria for Rett syndrome (RTT) but lacking a mutation in MECP2 , the gene predominantly associated with this disorder, were provided additional genetic testing. This testing revealed pathogenic mutations in a gene not previously associated with RTT, CTNNB1 , mutations in which lead to an autosomal dominant neurodevelopmental disorder affecting cell signaling and transcription factors as well as a likely pathogenic mutation in the WDR45 gene, which is associated with developmental delay in early childhood and progressive neurodegeneration in adolescence or adulthood related to iron accumulation in the globus pallidus and substantia nigra. These two individuals are described in relation to previous reports linking multiple other genes with RTT failing to show an MECP2 mutation. These individuals underscore the need to pursue additional molecular testing in RTT when a mutation in MECP2 is not detected.