This episode is based partly on the 'Gifted' story arc from 'Astonishing X-Men' (originally written by Joss Whedon) and involves a so-called "cure" for mutants. This story line was also used for the third 'X-Men' movie, which was the first screen appearance of the character Angel, who, perhaps not coincidentally, plays a major role in this episode despite not appearing in the original comics story.