In the development of biosynthesis, numerous logic pathways have been designed to fullfil certain functions. However, all these imaginative designs lack an important feature ---- once they are designed, no further alter can be done to the unit, which makes it difficult to be applied as fundamental units to large-scale complex systems.


Since many of us frequently deal with complex electric circuits, we get inspiration from an electronic component, PLD, to solve this problem.
A PLD or programmable logic device is used to build reconfigurable digital circuits. Unlike a logic gate, which has a fixed function, a PLD has an undefined function at the time of manufacture. Before the PLD can be used in a circuit it must be programmed, that is, reconfigured.
As is the fact, PLD serves as a milestone in the progress of digital technology and is now widespread in digital logic experiments. Thus we were motivated to apply it in cells and make logic-flexible, in another word, programmable gene circuits.