Clones

Clone Club is a term used by Cosima and has since also been used in the real world to refer to the group of clones. The supposed "first rule" of the club is that outsiders, particularly non-clones, cannot know about the clones' existence. However, since Sarah's joining of clone club, several people are in the know.

Although clones are supposed to be biologically identical, the clones in the show have differences in some portions of their genome. These portions have certain encoded, encrypted messages which translate into the clones' tag number, used to tell them apart, and the patent for their creation.

Apparently, the clones' mothers were either desperate for children and opted to undergo in-vitro fertilization, like Alison, Cosima and Beth's parents, or were possibly women who were approached by scientists posing as couples who wanted children of their own but needed a surrogate, like Sarah and Helena's apparent birth mother, Amelia.