Figure 1.

Schematic representation of a theoretical gene locus, highlighting various cis elements that contribute to the regulation of gene expression. Exons are indicated by rectangular boxes, with the protein-coding portions in black. Complexity of gene output can be achieved through use of alternative promoters and/or exons. Multiple cis-regulatory elements, indicated by ovals, control the quantitative and spatiotemporal specific expression. These elements may be at considerable distances from the promoter, either upstream or downstream, and are sometimes within or beyond an adjacent gene. The chromatin structure of the locus is determined by a combination of the activities of these cis elements and the wider chromosomal and nuclear environment. In some loci, the outermost cis elements may carry some boundary activity, isolating the specific chromatin structure of the gene domain from that of adjacent chromosomal segments.