Several randomly selected cDNA clones made from rat brain polyA+RNA have unusual properties. Although the cDNA inserts are 500-1,250 base pairs long, they hybridize to a 160-nucleotide RNA species that is present in brain but not in liver or kidney. Nucleotide sequence studies of two of the clones and hybridization studies show that a common 82-nucleotide sequence is responsible for the hybridization. The same sequence is located in the second intron of a rat growth hormone gene. These studies suggest that signals called "ID sequences" may be carried in the introns of genes and prescribe their tissue-specific expression.