Polycomb-binding sites within the bithorax complex were mapped by immunostaining of salivary gland polytene chromosomes . Polycomb binds to four DNA fragments, one in each of four successive parasegmental regulatory regions. Two models have been proposed to account for the dominant Fab-7 phenotype.
Evidence is provided that the boundary region contains an enhancer blocker element.
Deletions removing the PRE alone do not typically cause any visible phenotype as homozygotes. It is suggested that there are two phases to BX-C regulation. Once the activity states have been selected, BX-C regulation switches to the maintenance system. This is presumably accomplished by the assembly of a Polycomb group protein silencing complex on iab-7 . This is presumably accomplished by blocking interactions between the two domains.