Pelizaeus Merzbacher disease (PMD) is an X-linked recessive disorder of the central nervous system myelination caused by mutations involving the proteolipid protein gene (PLP). Early nystagmus and developmental delay, progressive pyramidal, cerebellar and dystonic signs as well as white matter changes in brain MRI are typical for PMD. The PLP gene can be affected by two major types of mutations. A duplication of the whole PLP gene is the most common mutation and results usually in the milder classical phenotype, whereas point mutations in PLP gene often result in the rarer and more severe connatal form of PMD. The PLP protein is a higly conserved across species and is identical in human, mouse and rat. We describe a 13-year-old Czech boy with an early and severe developmental delay. His maternal uncle died at the age of one year and was also early and severely psychomotoricly retarded. The patient was the first child of healthy unrelated parents born after an uneventful pregnancy and delivery in 1988. Hyperbilirubinemia and bronchopneumonia and early stridor complicated his neonatal period. Diffuse hypotonia, nystagmus, psychomotor retardation, visual and hearing impairment have been observed in the patient since the age of 6 weeks. White matter abnormalities, cortical and periventricular atrophy were detected by MRI at the age of 6 and 11 years, respectively. Despite these signs and results an accurate clinical diagnosis was unclear until the age of 11 years. Last neurological examination in 1999 showed no nystagmus anymore, but extremely dystrophic limbs, truncal deformation, due to severe scoliosis, tetraplegia with hyperreflexia in C5C7 and areflexia L2S2 and positive pyramidal signs. The boy had no visual or speech contact. DNA tests followed the clinical suspicion for PMD. At first, duplication of PLP gene was excluded by quantitative comparative PCR. Direct sequencing of PLP gene detected a novel mutation in exon 6, a missense mutation 725C-->A (Ala242Glu) in the patient and in his mother and later also in his maternal grandmother. The same codon, but to valine (Ala242Val) is mutated in jimpy(msd) mouse, which is the frequently used animal model for PMD. Prenatal diagnosis for the next pregnancy has been offered to the family. The patient died recently at the age of 13 years due to respiratory failure. Our results support the data on the importance of this conserved amino acid alanine at codon 242.