Although a limited number of patients have been described, it seems that this hypomyelinating leukodystrophy has well-defined clinical (i.e., the onset of psychomotor delay and/or nystagmus evolving into ataxic-spastic syndrome), genetic (i.e., mutations in the exon 3B or intron 3 of PLP1), and neuroradiological (i.e., involvement of early myelinated structures) features. The gene discussed is PLP1; the disease is pathologic nystagmus.