Non-Standard Residue

Any residue not included in the list of Standard Residues is considered Non-Standard.

Examples:

• SEP and TPO in 1bkx
• 1MA, 2MG, 5MC, 5MU, 7MG, H2U, M2G, OMC, OMG, PSU, and YG in 1evv
• MSE in 2ab5

Although phosphoserine and phosphothreonine are given as non-standard residues SEP and TPO in 1bkx, and alternative convention may also be followed. For example, phosophserine and phosphothreonine in 1apm are given and SER and THR plus PO3 groups.

The former scheme of designating modified standard nucleotides with plus signs (+A, +C, +G, +I, +T, +U) was discontinued in the PDB remediation project, effective August 1, 2007.