Congenital Hypomyelination (CH) is the most severe demyelinating form of Hereditary Motor and Sensory Neuropathies and manifests at birth in human. Some subtypes of CH are due to dommant mutations in the gene coding for PO glycoprotein, which fiinctions as a homophilic adhesion protein, responsible for compaction of opposing myelin lamellae. By homologous recombination in ES cells, we have generated a mouse containing a nonsense mutation m the intracellular portion of PO (Q215X) that, in the heterozygous state, is associated with CH neuropathy in humans.