  It is well known, although sometimes ignored, that not only the d=5 but also
d=6 proton decay depends on fermion mixings. In general we study carefully the
dependence of d=6 decay on fermion mixings using the effective operator
approach. We find that without specifying a theory it is impossible to make
clear predictions. Even in a given model, it is often not possible to determine
all the physical parameters. We point out that it is possible to make a clear
test of any grand unified theory with symmetric Yukawa couplings. We discuss in
some detail realistic theories based on SU(5) and SO(10) gauge symmetry.
