A cDNA minilibrary in pUC18 has been generated from 3-month-old rat brains. Two hundred clones were randomly selected and sequenced. Comparison with a nucleic acid and protein data bank revealed a number of cDNA fragments not homologous to any published sequence. Northern blot analyses of some of these clones yielded 5 brain-specific cDNA fragments. The full-length cDNA for one of these clones has been isolated and completely sequenced. It corresponds to an mRNA of about 1,500 nucleotides, which is present in brain and, to a lesser extent, in heart and skeletal muscle. Its expression is developmentally regulated in the rat brain from 14-day embryos to 3-month-old adults. A very recent comparison with the EMBL nucleotide sequence data bank showed that this mRNA codes for a brain-specific snRNP-associated protein.