From 2007.igem.org

As shown in Figure 1, promoter with a so-called "Dual-Repressed Operator" in the downstream can be used as a NOR gate. However, we faced a dilemma when integrating these dual-repressed NOR Gates into an actual system. It was because that most of the components in the system require wires exempt from interference while these NOR gates just in the contrary manner take the advantage of the mentioned interference. Therefore, the Dual-Repressed Operator should be carefully selected from the Repression Matrix.