Reusing an FM-index
  Intuitively, if two strings $S_1$ and $S_2$ are sufficiently similar and we already have an FM-index for $S_1$ then, by storing a little extra information, we should be able to reuse parts of that index in an FM-index for $S_2$. We formalize this intuition and show that it can lead to significant space savings in practice, as well as to some interesting theoretical problems. 
Djamal Belazzougui, Travis Gagie, Simon Gog, Giovanni Manzini and   Jouni Sir\'en
cs.DS