def normalize_sequence_to_blosum_characters(seq):
  """Make substitutions, since blosum62 doesn't include amino acids U and O.

  We take the advice from here for the appropriate substitutions:
  https://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/multalignviewer/multalignviewer.html

  Args:
    seq: amino acid sequence. A string.

  Returns:
    An amino acid sequence string that's compatible with the blosum substitution
    matrix.
  """
  return seq.replace('U', 'C').replace('O', 'X')