From 2006.igem.org

Abstract. We report here the design and implementation in vivo of a gene circuit that can count up to 4. In essence, it uses two toggle switches, each storing 1 bit, to keep track of the 4 states. The design of the counter is highly modular, in the hope that it can be included as a unit in larger circuits, but also combined with similar units to keep track of a much larger number of states, up to (2^(n+1)) with n units. To facilitate further development and integration to other projects, the counter is available in form of BioBricks.

Introduction

Counter Design

Methods

Results and Discussion