Bond cleavages in MS/MS fragmentation. The direct cleavage of peptide bonds results in b ions (containing the N-terminus of the peptide) and y ions (containing the C-terminus of the peptide). The other backbone bond cleavages result in a, x, c, and z ions. The breakage of two peptide bonds simultaneously gives rise to internal fragments along with b and y ions. Another common type is an immonium ion, comprised of an internal fragment of just one residue, formed by a combination of a-type and y-type cleavages.