From 2007.igem.org

Figure 1 Sketch map of Dual-Repressed Operator.

Figure 2 Best NOR, an example of Dual-Repressed Operators.

As shown in Figure 1 and 2, there may be two or more kinds of repressors that can both bind on one kind of operator tightly. A promoter with this so-called "Dual-Repressed Operator" in the downstream can be used as a well-performing 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 (Figure 3). (more details about this Matrix)

Remark: that best NOR above contains operator O11, which can be repressed by more than 3 kinds of repressors. It is just like the "wire OR" in electronics, see also the Logic Abstract in the right of Figure 1.