One male patient, T17133, presented with epileptic encephalopathy and was found to carry a de novo USP9X mutation (c.3034T>C, p.Ser1012Pro) at a highly conserved residue (GERP 5.62) that was predicted to be possibly damaging by PolyPhen (0.898). Here, USP9X is linked to Epileptic encephalopathy.