The methyl ester produced by the action of PCMT1 on L-isoaspartyl and D-aspartatyl amino acids subsequently undergoes spontaneous transformation to L-succinimide, which further spontaneously hydrolyses to generates L-aspartyl residues or L-isoaspartyl residues (Murray & Clarke 1986, Johnson et al. 1987, Galletti et al. 1988, Lowenson & Clarke 1992). This repair process helps to maintain overall protein integrity. When PCMT1 is not present in cells, the abnormal aspartyl residues accumulate. Pcmt1 knockout mice exhibit fatal progressive epilepsy (Yamamoto et al. 1998).