  We study the fluctuation properties of the asymmetric simple exclusion
process (ASEP) on an infinite one-dimensional lattice. When $N$ particles are
initially situated in the negative region with a uniform density $\rho_-=1$,
Johansson showed the equivalence of the current fluctuation of ASEP and the
largest eigenvalue distribution of random matrices. We extend Johansson's
formula and derive modified ensembles of random matrices, corresponding to
general ASEP initial conditions. Taking the scaling limit, we find that a phase
change of the asymptotic current fluctuation occurs at a critical position.
