Red "X"

In the overarching themes of the novel, the X is a visual representation of the unethical and subhuman treatment of the poor by the Republic. It acts as a marker to segregate the sick from the healthy, which in the case of the Republic cities serves to segregate the poor from the wealthy. It also holds historical symbolism, as a red X was used to mark plague victims’ residences in Europe during the epidemic of the Black Plague.

In the narrative, the red "x" literally symbolizes biological experimentation and danger, not unlike a biohazard symbol. The red “X” in the novel is a marking painted on the doors of plague victims in the slum sectors of the Republic. Republic patrols painted the X on Day’s door before they killed his family and took Eden, and he comes to learn that it signifies biological experimentation...