Protein-L-isoaspartate (D-aspartate) O-methyltransferase (PCMT1, PIMT EC 220.127.116.11) transfers the methyl group from S-adenosyl-L-methionine (AdoMet) to the alpha side-chain carboxyl group of L-isoaspartyl and D-aspartatyl amino acids (Murray & Clarke 1986, Johnson et al. 1987, Galletti et al. 1988, Lowenson & Clarke 1992, The resulting methyl ester (MetAsp) undergoes spontaneous transformation to L-succinimide, which spontaneously hydrolyses to generates L-aspartyl residues or L-isoaspartyl residues. 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).