The most common de novo MECP2 mutations in patients with RTT are R106W, R133C, T158M, R168X, R255X, R270X, R294X and R306C, which are responsible in total for about 60% of RTT cases and C-terminal deletions (9%) and large deletions (8.5%) [12,13,14]. The gene discussed is MECP2; the disease is Rett syndrome.