NEURONAL ceroid lipofuscinoses (NCL) represent a group of common progressive encephalopathies of children which have a global incidence of 1 in 12,500 (ref. 1). These severe brain diseases are divided into three autosomal recessive subtypes, assigned to different chromosomal loci^2á¤-4. The infantile subtype of NCL (INCL), linked to chromosome 1p32, is characterized by early visual loss and rapidly progressing mental deterioration, resulting in a flat electroencephalogram by 3 years of age; death occurs at 8 to 11 years⁵, and characteristic storage bodies are found in brain and other tissues at autopsy⁶. The molecular pathogenesis underlying the selective loss of neurons of neocortical origin has remained unknown. Here we report the identification, by positional candidate methods, of defects in the palmitoyl-protein thioesterase gene in all 42 Finnish INCL patients and several non-Finnish patients. The most common mutation results in intracellular accumulation of the polypeptide and undetectable enzyme activity in the brain of patients.