Fig. 6. Models for regulatory interdependence among closely linked genes. Regulatory interdependence ‘relays’ along neighboring genes and imposes organizational rigidity to closely linked genes or regions of chromosomes. Three closely linked neighboring genes (P2-P4) with their regulatory enhancers (E2-E3) are diagrammed. The specificity between the enhancers and their cognate promoters depends on the neighboring regulatory elements (E, enhancers; P, promoters; In, insulators), owing to the sharing and competing nature of the interactions between these elements (arrows). Changes in the relative positioning or regulatory capacity of any one component element, e.g. change of P4, a non-competitive promoter for E2 to P1, a highly competitive promoter for E2 (see purple arrow), will influence the regulatory outcome of neighboring interactions and in turn interactions further away thereby linking the entire genomic region into one regulatory, organizational and evolutionary unit.