The synuclein cDNA was previously cloned from Torpedo califonica using an antiserum against purified synaptic vesicles. Here we show that this gene hybridizes to several clones in a rat brain cDNA library. These clones represent cDNAs coding for different proteins, thus determining a family. These proteins display similar organization to the Torpedo protein. The homology resides within the repetition of 7 amino acids. The diversity is generated by alternative splicing as suggested by Southern analysis, as well as by sequence analysis. These proteins are specifically expressed in the rat brain. In situ hybridization shows expression of synuclein mRNAs in discrete areas of the brain, the hippocampus is the most intensely stained area. This localization is reminiscent of those molecules involved in phosphoinositols second messenger pathways. Synuclein proteins are present in rat brain in dual form--a small form which is soluble and a large form which is associated with synaptosomal membranes in a regulated fashion.