def merge_proteins(proteins):
    """ Merge all isozymes that catalyze a given reaction.
    Automatically removes all isozymes with missing score.

    Args:
        proteins (pandas.Series): list of proteins

    Returns:
        str: boolean rule

    """
    proteins = set(proteins.dropna())
    if not proteins:
        return None
    gpr_str = ' or '.join(sorted(proteins))
    if len(proteins) > 1:
        return '(' + gpr_str + ')'
    else:
        return gpr_str
