Gene expression is tightly controlled at multiple levels by dif-ferent categories of cis-regulatory elements(CREs)and trans-acting factors(TAFs).Two different regulators might have additive or even synergistic effects,enabling robust repression or activation of target gene expression(i.e.,the“fail-safe”regulation).Alterna-tively,two regulators might antagonistically regulate the same tar-gets,which seems futile,but could stabilize gene expression.As a transcript could be regulated by multiple mechanisms,the combi-natorial use of different categories of CREs and TAFs might be prevalent in modulating the function and fate of an mRNA[1,2].