Extended uppercase IUPAC protein single letter alphabet including X etc.

In addition to the standard 20 single letter protein codes, this includes:

• B = “Asx”; Aspartic acid (R) or Asparagine (N)
• X = “Xxx”; Unknown or ‘other’ amino acid
• Z = “Glx”; Glutamic acid (E) or Glutamine (Q)
• J = “Xle”; Leucine (L) or Isoleucine (I), used in mass-spec (NMR)
• U = “Sec”; Selenocysteine
• O = “Pyl”; Pyrrolysine

This alphabet is not intended to be used with X for Selenocysteine (an ad-hoc standard prior to the IUPAC adoption of U instead).