def _findUniqueMappingValues(mapping):
    """Find mapping entries that are unique for one key (value length of 1).

    .. Note:
        This function can be used to find unique proteins by providing a
        peptide to protein mapping.

    :param mapping: dict, for each key contains a set of entries
    :returns: a set of unique mapping values
    """
    uniqueMappingValues = set()
    for entries in viewvalues(mapping):
        if len(entries) == 1:
            uniqueMappingValues.update(entries)
    return uniqueMappingValues