Prior to the emergence of life, it is believed that only L-amino acids were selected for formation of protein and that D-amino acids were eliminated on the primitive Earth. Whilst homochirality is essential for life, the occurrence of proteins containing D-beta-aspartyl (Asp) residues in various tissues from elderly subjects has been reported recently. Here, we demonstrate the presence of a D-beta-Asp-containing protein in the cardiac muscle of heart, blood vessels of the lung, chief cells of the stomach, longitudinal and circular muscle of the stomach, small intestine and large intestine. Since the D-beta-Asp residue occurs through a succinimide intermediate, this isomer may potentially be generated in proteins more easily than initially thought. Formation of the D-beta-Asp residue in proteins may be related to stress.