  In string theory, massless particles often originate from a symmetry breaking
of a large gauge symmetry G to its subgroup H. The absence of dimension-4
proton decay in supersymmetric theories suggests that (\bar{D},L) are different
from \bar{H}(\bar{\bf 5}) in their origins. In this article, we consider a
possibility that they come from different irreducible components in
$\mathfrak{g}/\mathfrak{h}$. Requiring that all the Yukawa coupling constants
of quarks and leptons be generated from the super Yang--Mills interactions of
G, we found in the context of Georgi--Glashow H=SU(5) unification that the
minimal choice of G is E_7 and E_8 is the only alternative. This idea is
systematically implemented in Heterotic String, M theory and F theory,
confirming the absence of dimension 4 proton decay operators. Not only H=SU(5)
but also G constrain operators of effective field theories, providing
non-trivial information.
