Boundary bypass activity in the region of the bithorax complex is position dependent and regulated.

Expression of () in abdominal segments A5A8 is controlled by four regulatory domains, . Each domain has an initiator element (which sets the activity state), elements that maintain this state and tissue-specific enhancers. To ensure their functional autonomy, each domain is bracketed by boundary elements (, , and ). In addition to blocking crosstalk between adjacent regulatory domains, the boundaries must also have bypass activity so the relevant regulatory domains can 'jump over' intervening boundaries and activate the promoter. In the studies reported here we have investigated the parameters governing bypass activity. We find that the bypass elements in the and boundaries must be located in the regulatory domain that is responsible for driving expression. We suggest that bypass activity may also be subject to regulation.