Background: Mutations in the cyclin-dependent kinase-like 5 gene (CDKL5) located in the Xp22 regionhave been shown to cause a subset of atypical Rett syndrome with infantile spasms or earlyseizures starting in the first postnatal months. Methods: We performed mutation screening of CDKL5 in 60 female patients who had been identifiedas negative for the methyl CpG-binding protein 2 gene (MECP2) mutations, but who hadcurrent or past epilepsy, regardless of the age of onset, type, and severity. All the exons in theCDKL5 gene and their neighbouring sequences were examined, and CDKL5 rearrangementswere studied by multiplex ligation-dependent probe amplification (MLPA). Results: Six previously unidentified DNA changes were detected, two of which were disease-causingmutations in the catalytic domain: a frameshift mutation (c.509_510insGT;p.Glu170GlyfsX36) and a complete deletion of exon 10. Both were found in patients withseizures that started in the first month of life. Conclusions: This study demonstrated the importance of CDKL5 mutations as etiological factors inneurodevelopmental disorders, and indicated that a thorough analysis of the CDKL5 genesequence and its rearrangements should be considered in females with Rett syndrome-likephenotypes, severe encephalopathy and epilepsy with onset before 5 months of age. Thisstudy also confirmed the usefulness of MLPA as a diagnostic screening method for use inclinical practice.