On the opposite end of the spectrum are the Proletheans. These guys are basically against everything that the Neolution movement stands for, and believe that the clones—with the exception of Helena—are all abominations that should be killed.

As a result, they've trained Helena from the time she was a baby to believe in their way of thinking, and to think that she is the "original" clone; the rest (Alison, Cosima, Beth, Katja, Jennifer, and Rachel), they say, are all mere images of her.

Helena initially believes that Sarah is as O.G., but she quickly realizes that the two have a special bond because they're actually twins, separated at birth.

While we don't know much about the Proletheans yet (expect this to change in season two), it's clear that the group is dangerous. They're radical extremists who have no issue resorting to murder in order to ensure their vision of a Neolutionist-free society come true.

So, basically, don't fuck with them.