'Identifier' or ID sequences are present in 62% of the RNA polymerase II and III transcripts made in vitro from brain nuclei but in fewer than 4% of the transcripts made from the nuclei of other tissues. An homologous 160-nucleotide cytoplasmic poly(A)+ RNA species, BC1, and a smaller species, BC2, are located in vivo exclusively in neural tissues. Cloned ID sequences are polymerase III templates in vitro. Our data suggest a model in which brain-specific polymerase III transcription of ID sequences located in introns of brain genes activates those genes in a primary manner for polymerase III transcription.