def _add_sequence(self, pdbID, chainID, sequence):
        '''This is a 'private' function. If you call it directly, call _find_identical_sequences() afterwards to update identical_sequences.'''
        pdbID = pdbID.upper()
        self[pdbID] = self.get(pdbID, {})
        self[pdbID][chainID] = sequence
        self.sequences.append((pdbID, chainID, sequence))
        if not self.unique_sequences.get(sequence):
            self.unique_sequences[sequence] = visible_colors[len(self.unique_sequences) % len(visible_colors)]
        self.identical_sequences = None