Should be called after get_mutations.

def get_corresponding_chains(self, from_pdb_id, from_chain_id, to_pdb_id):
        '''Should be called after get_mutations.'''
        chains = self.chain_map.get(from_pdb_id, {}).get(from_chain_id, {}).get(to_pdb_id, [])
        return sorted(chains)