ALPHA-SYNUCLEIN has been implicated in the pathogenesis of Parkinsonsis disease. To investigate the role of α-synuclein in the brain, the cDNA clone encoding the mouse cognate of the human α-synuclein was isolated from a mouse brain cDNA library. The open reading frame coded for 140 amino acids that share 95% identity with human α-synuclein. Northern blot analysis showed that α-synuclein mRNA was primarily expressed in brain and spleen of adult mouse. In situ hybridization histochemistry revealed the highest expression of α-synuclein mRNA in the hippocampal formation and neocortex of the adult mouse. α-synuclein mRNA expression in the brain was first observed in the hippocampus and neocortex on postnatal day 1. Levels of α-synuclein mRNA in these forebrain areas were nearly maximal at postnatal day 7 and remained relatively high until the adult stage. α-synuclein mRNA was expressed in the liver transiently during embryogenesis.