Possible models for the selection of an indirect target. For both panels, site A is the direct target of a transcription factor (TF) "A" and B is the indirect site. The figure illustrates two models for the detection of site B, in addition to site A. Panel A. Sequential model. One of the gene products activated by site A ('C gene' in the figure) targets site B. Panel B. Bystander model. Sites A and B coexist in the same promoter and may cooperatively function to activate the target promoter. Another possibility is that site B is not involved in the gene activation by TF "A" but is involved in a distinct signaling pathway, resulting in site A and B, having only a biological relationship. A possible example of this latter case is the coexistence of a site for an environmental response and for tissue-specific expression (e.g., light response and leaf-specific expression).