Figure 7
Three types of boundary elements. (A) Boundary element intercepts the effect of an enhancer to the nearby promoter. (B) Boundary element between heterochromatin and euchromatin serves as a barrier against the spreading of heterochromatin. (C) Boundary elements residing in the BX-C region regulate the three homeotic genes to ensure the correct level and pattern of expression, thereby making possible proper segmentation in the Drosophila embryo.