def METHOD_NAME(genes, ref_path):
    """
    Transfer mouse genes to their human homologous genes.
    If one-to-many, use the first. If mouse gene not in the reference data, mark it as 'NotAvailable'.
    :param genes: list of mouse genes (hgnc symbols)
    :param ref_path: path of the reference homologous gene relationship file.
    :return: list of human homologous
    """
    ref_separator = get_separator(os.path.splitext(ref_path)[-1])
    ref_df = pd.read_csv(ref_path, sep=ref_separator)
    ref_df = ref_df.drop_duplicates(subset='genesymbol_source', keep='first')
    ref_dict = dict(ref_df.loc[:, ['genesymbol_source', 'genesymbol_target']].values)
    genes_human = [ref_dict[x] if x in ref_dict.keys() else 'NotAvailable' for x in genes]
    human_genes_to_mouse = {ref_dict[x]: x for x in genes if x in ref_dict}
    return genes_human, human_genes_to_mouse