MHC class I molecules predominantly bind to peptides derived from a cytosolic pool of polypeptides. Little is known about the nature of the polypeptides that serve as substrates for peptidogenic cytosolic proteases. We propose that a significant source of self and viral peptides are defective ribosomal products (DRiPs), which consist of prematurely terminated polypeptides and misfolded polypeptides produced from translation of bona fide mRNAs in the proper reading frame. DRiPs are produced entropically, due to the inevitable imperfections inherent to protein synthesis or folding. To accelerate recognition of cells harboring intracellular parasites such as viruses, DRiP formation may be enhanced by changes in the cellular physiology induced by infection or by exposure of cells to cytokines released at the site of inflammation.