def check_coverage_single_pos(cov_df, variants_bed_sub, rsid, threshold = 20):
    """
    Checks the coverage of a single position. If the coverage is greater 
    than the threshold it returns a "PASS" flag, otherwise returns "FAILED". 
    For the HLAs always returns "CHECK".

    Parameters
    ----------
    cov_df: Pandas Dataframe
        Dataframe which stores the coverage every position queried.
    variants_bed_sub: Pandas Dataframe
        Subset of the variants_table just with the genes for which we have
        called more than one position.
    rsid : String
        The rsID of the instance.
    threshold: Integer
        Threshold to assign the flag.

    Returns
    -------
    found: String
        The major star allele.
    """
    position = variants_bed_sub.loc[variants_bed_sub.rsid == rsid, "end"].item()
    if position > threshold:
        flag = "PASS"
    else:
        flag = "FAILED"
    return flag