An agent that causes change in the activity of a structure

In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level.

Explanation:

precisely the author keeps the interest of readers not describing clearly the   kind of relationship in giselle's family. by doing this, readers had to imagine and assume how it was.   instead, the author described better scenes with dark star and the influence of books about horses in giselle's life and actions.   also the fact that she broke her arm but not referring to it in detail leads the readers to imagine and guess what could have occurred.