MECP2 and atypical Rett syndrome: One possible explanation for the conflicting findings in the two studies is the different clinical presentations of Rett syndrome and Phelan–McDermid syndrome in humans; while patients with SHANK3 mutations typically display signs of neurological impairments at birth (Phelan & McDermid, 2012), patients with MECP2 mutations typically present after 12–16 months (Neul et al., 2010).