Cathepsin is activated early by MNV-1 infection. Fragmentation of tryptic peptides in an ion trap mass spectrometer was used to identify proteins labeled by a poly-caspase probe. This spectrum shows one of two tryptic peptides identified from Cathepsin. Amino acid sequence is shown in reverse order (C to N terminus) to match the Y ion series highlighted by the vertical bars. Ten B ions are also present but not marked. The match was highly significant in Mascot (p = 4.7 × 10-10) and Phenyx (p = 2.0 × 10-57) protein identification searches.