STXBP1 and early-infantile DEE: Excluding large rearrangements detected by MLPA, 29 patients had mutations in RTT genes and 90% in MECP2. From 18 patients with mutations in RTT-like genes, it is remarkable that the majority of these mutations were in the STXBP1 gene, which is associated with early infantile epileptic encephalopathy (EIEE4)20, and the TCF4 gene, which is associated with Pitt–Hopkins syndrome21.