[en] Using the generalised AdS/CFT correspondence, we show that there are certain ten-dimensional differentiable manifolds such that string theory on such a manifold is unstable [to the emission of 'large branes'] no matter what the metric may be. The instability is thus due to the [differential] topology of the manifold, not to any particular choice of its geometry. We propose a precise criterion for this 'topology selection mechanism', and prove it in many cases. The techniques employed may be useful in more general cases. (author)