This interactive activity from NOVA examines how mutations in different regions of an organism's DNA affect gene expression—whether genes are turned on or off during the organism's development. An animated model depicts two DNA segments in the nucleus of a fertilized egg. Each segment contains a coding gene, enhancers in the noncoding regulatory region, and other DNA not associated with the gene. As the egg develops, each cell produces its own proteins. Whether and where these proteins bind to the DNA determines gene expression. Mutations can change how receptors in each of the regions permit or prevent such binding. This, in turn, influences which traits are displayed.