We believe that this work, along with the review of the cases described so far, allows us to deepen our knowledge of the clinical aspects of twins with RTT syndrome, particularly with regard to epilepsy, and to focus attention on the variability of allelic expression of the MECP2 gene in patients with a very severe phenotype, which needs, however, further, and more in-depth genetic investigations. The gene discussed is MECP2; the disease is epilepsy.