MECP2 and atypical Rett syndrome: Along these lines, when the L1 qPCR assay was applied to brain tissue obtained from i) Rett syndrome (RTT) patients, where mutations in the L1 transcriptional repressor MeCP2 (Table 1) [27, 75, 228, 229] cause a severe neurodevelopmental disorder, and ii) an MeCP2-mutant RTT mouse model, significant L1 copy number gain was observed in either species when L1 DNA content was measured at ORF2, when compared to controls [27].