MECP2 and Rett syndrome: More than 99% of RTT cases are sporadic, and their pathogenic MECP2 variants are mainly de novo and of paternalorigin.16,17 Therefore, we wondered whether paternalembryonic mosaicism (affecting both somatic and germline tissues) or clonalexpansion (germline-specific) mosaicism is the origin of MECP2 de novo variants in RTT patients.