  We study the phase transition between G-instantons and D3-branes. A
G-instanton is a classical solution to the self-dual equation of the M/F-theory
four-form field strength G in the complex fourfold. This phase transition is
dual to that between small instantons and 5-branes in the heterotic string.
Using G as a background gauge flux, we may dynamically control the gauge
symmetry breaking, connect between different vacua of F-theory and understand
D3-branes in terms of group-theoretical quantities. We also discuss the
resulting chirality change and preservation of anomaly freedom.
