def _merge_subexon_columns(subexons_df):
    """Create the SubexonID/SubexonIDCluster of the merged subexons."""
    merged_id = ''
    merged_protein = ''
    merged_dna = ''
    for index in range(1, subexons_df.shape[0]):
        rowi = subexons_df.iloc[(index - 1), :]
        rowj = subexons_df.iloc[(index), :]
        if not merged_id:
            merged_id = rowi['SubexonIDCluster'] + '_' + rowj[
                'SubexonIDCluster'].split('_')[-1]
        else:
            merged_id = merged_id + '_' + rowj['SubexonIDCluster'].split('_')[
                -1]
        if not merged_protein:
            merged_protein = rowi['SubexonProteinSequence'] + rowj[
                'SubexonProteinSequence']
        else:
            merged_protein = merged_protein + rowj['SubexonProteinSequence']
        if not merged_dna:
            merged_dna = rowi['SubexonSequence'] + rowj['SubexonSequence']
        else:
            merged_dna = merged_dna + rowj['SubexonSequence']
    return merged_id, merged_dna, merged_protein
