def translate_genes(id_list=None, from_id=None, to_id=None, organism=None):
    """
    Pass a list of identifiers (id_list), the name of the database ('Entrez',
    'Symbol', 'Standard name', 'Systematic name' or a loaded crossreference
    database) that you wish to translate from, and the name of the database
    that you wish to translate to.
    """

    ids = set(id_list)
    # Initialize set of identifiers not found by this translate_genes method.
    not_found = set()
    from_ids = None  # Get the map of from_ids to the gene pks

    if organism is not None:
        gene_objects_manager = Gene.objects.filter(
            organism__scientific_name=organism)
    else:
        gene_objects_manager = Gene.objects

    if (from_id == 'Entrez'):
        int_list = []
        for x in ids:
            try:
                int_list.append(int(x))
            except(ValueError):
                not_found.add(x)
        ids = set(int_list)
        from_ids = gene_objects_manager.filter(entrezid__in=ids).values_list(
            'entrezid', 'id')
    elif (from_id == 'Systematic name'):
        from_ids = gene_objects_manager.filter(
            systematic_name__in=ids).values_list('systematic_name', 'id')
    elif (from_id == 'Standard name'):
        from_ids = gene_objects_manager.filter(
            standard_name__in=ids).values_list('standard_name', 'id')
    elif (from_id == 'Symbol'):
        # If standard_name exists, symbol will be standard_name; otherwise
        # symbol will be systematic_name.
        from_ids = gene_objects_manager.annotate(
            symbol=Coalesce('standard_name', 'systematic_name')).filter(
                symbol__in=ids).values_list('symbol', 'id')
    else:  # a crossreference db?
        xrdb = CrossRefDB.objects.get(name=from_id)
        from_ids = CrossRef.objects.filter(crossrefdb=xrdb).values_list(
            'xrid', 'gene__id')

    # Dictionary that maps from type ID passed by user to gene__id.
    from_id_map = {}
    gene_ids = []
    for item in from_ids:
        from_id_map[item[0]] = item[1]
        gene_ids.append(item[1])

    # Now let's figure out what we need to go to:
    to_ids = None
    if (to_id == 'Entrez'):
        to_ids = Gene.objects.filter(id__in=gene_ids).values_list(
            'id', 'entrezid')
    elif (to_id == 'Systematic name'):
        to_ids = Gene.objects.filter(id__in=gene_ids).values_list(
            'id', 'systematic_name')
    elif (to_id == 'Standard name'):
        to_ids = Gene.objects.filter(id__in=gene_ids).values_list(
            'id', 'standard_name')
    elif (to_id == 'Symbol'):
        # If standard_name exists, symbol will be standard_name; otherwise
        # symbol will be systematic_name.
        to_ids = Gene.objects.annotate(
            symbol=Coalesce('standard_name', 'systematic_name')).filter(
                id__in=gene_ids).values_list('id', 'symbol')

    else:  # A crossreference db?
        xrdb = CrossRefDB.objects.get(name=to_id)
        to_ids = CrossRef.objects.filter(crossrefdb=xrdb).values_list(
            'gene__id', 'xrid')

    to_id_map = {}
    for item in to_ids:
        if not item[0] in to_id_map:
            to_id_map[item[0]] = [item[1], ]
        else:
            to_id_map[item[0]].append(item[1])

    from_to = {}
    for item in ids:
        try:
            gene_id = from_id_map[item]
        except KeyError:
            not_found.add(item)
            continue
        to_id = to_id_map[gene_id]
        from_to[item] = to_id
    from_to['not_found'] = list(not_found)

    return from_to
Pass a list of identifiers (id_list), the name of the database ('Entrez',
    'Symbol', 'Standard name', 'Systematic name' or a loaded crossreference
    database) that you wish to translate from, and the name of the database
    that you wish to translate to.