def _max_idx(df, column=0):
    """
    Returns probe ID with max index in `df`

    Parameters
    ----------
    df : (P, 1) pandas.DataFrame
        Dataframe with `P` rows indicating distinct probes and one column
        containing summary statistic of probe expression
    column : str, optional
        Column name from which to extract the max index. If not specified uses
        the first numerical column.

    Returns
    -------
    probe_id : str
        ID of probe selected as representative for given gene
    """

    return df.idxmax()[column]