def _map_agent_mod(self, agent, mod_condition):
        up_id = _get_uniprot_id(agent)
        if not up_id:
            logger.debug("No uniprot ID for %s" % agent.name)
            return None
        if mod_condition.position is None or mod_condition.residue is None:
            return None
        mapped_site = \
            self.map_to_human_ref(up_id, 'uniprot',
                mod_condition.residue,
                mod_condition.position,
                do_methionine_offset=self.do_methionine_offset,
                do_orthology_mapping=self.do_orthology_mapping,
                do_isoform_mapping=self.do_isoform_mapping)
        return mapped_site