def is_indel_len(mut_df):
    """Simply returns a series indicating whether each corresponding mutation
    is an indel.

    Parameters
    ----------
    mut_df : pd.DataFrame
        mutation input file as a dataframe in standard format

    Returns
    -------
    is_indel : pd.Series
        pandas series indicating if mutaitons are indels
    """
    is_indel = (mut_df['Reference_Allele'] == '-') | (mut_df['Tumor_Allele'
        ] == '-')
    return is_indel
