The authors report the unusual clinical and neurophysiologic features of a sporadic case of a boy carrying an 806delG mutation on the MECP2 gene. A 28- month- old boy was examined for severe developmental delay, seizures, microcephaly, breathing dysfunction, and spontaneous and evoked myoclonic jerks of upper limbs. Neurophysiologic study proved the corticalorigin of myoclonus; however, it was not associated with signs of cortical hyperexcitability. 3- Methoxy- 4- hydroxy- phenyleth- ylene glycol and valine concentrations were low in CSF. The authors report the unusual clinical and neurophysiologic features of a sporadic case of a boy carrying an 806delG mutation on the MECP2 gene. A 28- month- old boy was examined for severe developmental delay, seizures, microcephaly, breathing dysfunction, and spontaneous and evoked Neurophysiologic studies proved the corticalorigin of myoclonus; however, it was not associated with signs of cortical hyperexcitability. 3-Methoxy- 4-hydroxy- phenylethynyllene and valine concentrations were low in CSF.