A genetic module. Shown here is a genetic module as they are often used in the synthetic biology community. The first module, the Inverter, is usually placed under the control of some promoter, r1. When r1's activity is high, r2 is low, and when r1 is low, r2 is high, hence the inversion. The dotted line between promoter r1 and the Inverter module can be thought of as the flux of RNA polymerase. The connection between the Inverter module and the M1 module is also the same. Therefore, r2 controls the transcription rate of the genes in module M1.