  We exhibit a Cremona transformation of ${\bf P}^4$ such that the base loci of
the map and its inverse are birational to K3 surfaces. The two K3 surfaces are
derived equivalent but not isomorphic to each other. As an application, we show
that the difference of the two K3 surfaces annihilates the class of the affine
line in the Grothendieck ring of varieties.
