Therefore, in the present study, we performed overnight PSG recordings in RTT girls with MECP2 mutations to characterize sleep breathing abnormalities in RTT, to examine the hypotheses that (1) sleep breathing abnormalities in the individuals with RTT are present since early childhood, (2) sleep macrostructure between RTT girls with and without SDB is different, (3) the sleep breathing disturbances differ during REM and NREM sleep, and (4) the severity of SDB may aggravate with the cardinal clinical features’ severity in RTT. Here, MECP2 is linked to sleep apnea syndrome.