Updated project metadata. Using high­throughput mass spectrometry, we probed the six­reading­frame translation of human B cells’ transcriptome. We report that about 10% of MHC I­associated peptides (MAPs) derive from non­canonical reading frames. These so­called cryptic MAPs originate from allegedly non- coding genomic sequences and from out­of­frame translation. Their biogenesis and properties differ in many ways from those of conventional MAPs. Thus, cryptic MAPs come from very short proteins with atypical C­termini, and they are coded by transcripts bearing long UTRs selectively enriched in destabilizing elements. Cryptic MAPs increase the complexity of the immunopeptidome and represent an heretofore unexploited source of potential tumor­specific epitopes. In a more general context, the features of cryptic MAPs suggest that mRNA instability is instrumental in the biogenesis of MAPs. Associated RNA-seq accession: GSE67174 .