To date, 43 patients have been described with mutations in or involving the CDKL5 gene. The typical phenotype includes early-onset, often intractable epileptic seizures and severe mental retardation with very limited progress in psychomotor development. Most patients also show impaired social interaction with avoidance of eye-to-eye contact, and some clinical features reminiscent of Rett syndrome (RTT), including stereotypic hand movements, lack of purposeful hand use, acquired microcephaly, and generalized hypotonia. We report on the case of a 5-year-old girl with a de novo CDKL5 gene mutation who developed early puberty, which has not been described before.