  A problem studied in Systems Biology is how to find shortest paths in
metabolic networks. Unfortunately, simple (i.e., graph theoretic) shortest
paths do not properly reflect biochemical facts. An approach to overcome this
issue is to use edge labels and search for paths with distinct labels.
  In this paper, we show that such biologically feasible shortest paths are
hard to compute. Moreover, we present solutions to find such paths in networks
in reasonable time.
