The scanning of maturing mRNAs by ribosomes plays a key role in the mRNA quality control process. When ribosomes first engage with the newly synthesized mRNA, and if peptides are produced, is unclear, however. Here we show that ribosomal scanning of prespliced mRNAs occurs in the nuclear compartment, and that this event produces peptide substrates for the MHC class I pathway. Inserting antigenic peptide sequences in introns that are spliced out before the mRNAs exit the nuclear compartment results in an equal amount of antigenic peptide products as when the peptides are encoded from the main open reading frame (ORF). Taken together with the detection of intron-encoded nascent peptides and RPS6/RPL7-carrying complexes in the perinucleolar compartment, these results show that peptides are produced by a translation event occurring before mRNA splicing. This suggests that ribosomes occupy and scan mRNAs early in the mRNA maturation process, and suggests a physiological role for nuclear mRNA translation, and also helps explain how the immune system tolerates peptides derived from tissue-specific mRNA splice variants.MHC class I restricted antigen presentation | mRNA maturation | nuclear translation R NAs carrying premature termination codons or are recognized as defective in other aspects are prevented from further translation and disposed of by the nonsense-mediated decay (NMD) pathway (1). The detection of premature stop codons is mediated by the scanning ribosome; however, when the ribosomes first engage with the newly synthesized mRNA, and if this event results in the production of peptides, is unclear. Two observations from the field of immunology and the presentation of peptides on MHC class I molecules have highlighted some aspects relevant to the role of the ribosomes in the mRNA maturation process. The first observation is related to the detection of antigenic peptides originating from intron sequences, and the second is related to the observation that the synthesis of antigenic peptide substrates and full-length proteins are two spatiotemporarily distinct events (2-5).The presentation of antigenic peptides on MHC class I molecules allows CD8 + T cells to detect and eliminate cells in which the repertoire of peptide substrates has been altered owing to the presence of viruses or to changes in the presentation of endogenous antigens (6, 7). However, despite the key role of antigen presentation on MHC class I molecules in immune surveillance, the source of peptides for the MHC class I pathway is not yet known. Accumulating evidence indicates that the processing of alternative peptide substrates plays a major role, and that degradation products derived from full-length proteins have limited access to the MHC class I pathway (8-10). We recently demonstrated that pioneer translation products (PTPs) that form antigenic peptide substrates for the MHC class I pathway are produced by a translation event that is distinct from the canonical event giving rise to full-length proteins and does not require the cap-binding ...