def merge_subunits(genes):
    """ Merge list of protein subunit genes into complex

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

    Returns:
        str: boolean rule

    """
    genes = genes.dropna()

    if len(genes) == 0:
        return None
    else:
        protein = ' and '.join(sorted(genes))
        if len(genes) > 1:
            return '(' + protein + ')'
        else:
            return protein