Let Q be a finite quiver without oriented cycles, and let $\Lambda$ be the
corresponding preprojective algebra. Let g be the Kac-Moody Lie algebra with
Cartan datum given by Q, and let W be its Weyl group. With w in W is associated
a unipotent cell N^w of the Kac-Moody group with Lie algebra g. In previous
work we proved that the coordinate ring \C[N^w] of N^w is a cluster algebra in
a natural way.