def top_expression_effect(self, expression_levels):
        effect_expression_dict = self.effect_expression(expression_levels)
        if len(effect_expression_dict) == 0:
            return None
        def key_fn(effect_fpkm_pair):
            (effect, fpkm) = effect_fpkm_pair
            return (fpkm, multi_gene_effect_sort_key(effect))
        return max(effect_expression_dict.items(), key=key_fn)[0]