A Certain Magical Index, Volume 3 (A Certain Magical Index #11-15)

Mikoto Misaka is a Level Five esper - one of the most powerful in Academy City. She's so powerful, in fact, that her DNA has been harvested to create a series of clones - the "Sisters." But who is behind these clones, and why would they go to such lengths? The sinister truth is nothing she or Touma could have imagined, and it involves the terrifying Accelerator, the most powerful esper of all time!