DNA plasmid is shown with promoters pA and pB of genes gA and gB that, when transcribed, produce proteins A and B, respectively. The gene-promoter complex for species A(B) is denoted α(β) in Eq.(1). In this gene circuit, production of A inhibits B or production of B inhibits A. In Eq.(2), this is shown by having A bind to the gene-promoter complex of B, and vice-versa.