MECP2 and Rett syndrome: Thus, patients with Rett syndrome include cells with a MECP2 mutation (MECP2-MUT) on the Xi (XaMECP2 XiMECP-MUT; Xa: active X chromosome, Xi: inactive X chromosome) and cells with a MECP2 mutation on the Xa (XaMECP2-MUT XiMECP2) (Figure 1A).