def _single_sample_maybe_sufficient_reads(isoform1, isoform2, n_junctions,
                                          min_reads, case, letters='ab'):
    """Check if the sum of reads is enough compared to number of junctions

    Parameters
    ----------
    isoform1, isoform2 : pandas.Series
        Number of reads found for a single sample's isoform1 and isoform2
        junctions
    n_junctions : int
        Total number of junctions, also len(isoform1) + len(isoform2)
    min_reads : int
        Minimum number of reads per junction
    case : str
        English explanation with case number for why an event was rejected or
        not. Must contain curly braces ("{}") to place the letter into.
    letters : str, optional
        Cases could have multiple clauses, by default the case is "option a" or
         "option b" but other letters could be used. (default='ab')

    Returns
    -------
    isoform1, isoform2 : pandas.Series or None
        If the sum of reads on all isoforms is equal to or greater than the
        minimum number of reads times the number of junctions, then return the
        event. Otherwise, there are insufficient reads and the event is
        rejected.
    """
    if (isoform1.sum() + isoform2.sum()) >= (min_reads * n_junctions):
        # Case 5a: There are sufficient junction reads
        return isoform1, isoform2, '{case}: There are sufficient junction ' \
                                   'reads'.format(case=case.format(letters[0]))
    else:
        # Case 5b: There are insufficient junction reads
        return None, None, '{case}: There are insufficient junction ' \
                           'reads'.format(case=case.format(letters[1]))