The neuronal ceroid lipofuscinoses (NCLs) are a genetically heterogeneous group of progressive neurodegenerative disorders characterized by the accumulation of autofluorescent lipopigment in various tissues 1. Progressive epilepsy with mental retardation (EPMR, MIM 600143) was recently recognized as a new NCL subtype 2 (CLN8). It is an autosomal recessive disorder characterized by onset of generalized seizures between 5 and 10 years, and subsequent progressive mental retardation 3. Here we report the positional cloning of a novel gene, CLN8, which is mutated in EPMR. It encodes a putative transmembrane protein. EPMR patients were homozygous for a missense mutation (70C→ G, R24G) that was not found in homozygosity in 433 controls. We also cloned the mouse Cln8 sequence. It displays 82% nucleotide identity with CLN8, conservation of the codon harbouring the human mutation and is localized to the same region as the motor neuron degeneration mouse, mnd, a naturally occurring mouse NCL (ref. 4). In mnd/mnd mice, we identified a homozygous 1-bp insertion (267-268insC, codon 90) predicting a frameshift and a truncated protein. Our data demonstrate that mutations in these orthologous genes underlie NCL phenotypes in human and mouse, and represent the first description of the molecular basis of a naturally occurring animal model for NCL.