Dynamical $\mu$ and MSSM

We present the idea that the vacuum can choose one pair of Higgs doublets by making the $\mu$ parameter a dynamical field called {\it massion}. The {\it massion} potential leading to the dynamical solution is suggested to arise from the small instanton interaction when the gauge couplings become strong near the cutoff scale $M_s$ or $M_P$. One can construct supergravity models along this line. We also present an explicit example with a trinification model from superstring.