We have obtained the sequence of a cDNA for an abundant mRNA species that occursto a large extent as untranslated mRNP particles unable to interact with the translation apparatus in mouse tumor cell lines (1). The cDNA was isolated from a mouse L cell library in the Okayama-Berg vector (2). Primer extension with a synthetic oligonucleotide indicated that it is within 2-5 nucleotides of being full length. The sequence shows an open reading frame for 172 amino acids. A shorter reading frame (for 56 amino acids), starting 29 nucleotides downstream of the initiation AUG, is also possible. The sequence begins with a run of Ts followed by a GC-rich segment, a feature observed on the 5'terminus of the transcripts of some mouse ribosomal protein genes, that are also under translational control (3). A similar initial sequence and a downstream shorter reading frame also occur on another major mousemRNA (P40 mRNA) under translational control (1). Supported by NIH researchgrant GM17973.