In trio PKASD-18, which has one autistic and one neurotypical son, we identified a novel X-linked maternally inherited missense mutation, NM_003159.3:c.2626A > G; p.Ile876Val in CDKL5 (Cyclin-Dependent Kinase-Like 5), a known gene for an infantile epileptic encephalopathy and Rett syndrome-like disorder (MIM 300672), which, like Rett syndrome, affects girls almost exclusively. The gene discussed is CDKL5; the disease is Rett syndrome.