  In the Standard Model, no dim-5 $\Delta B \neq 0$ operators exist, so that
Planck-scale-induced proton decay amplitudes are suppressed by at least
$1/M^2_{P \ell}$. If the Standard Model is augmented by a light,
color-non-singlet boson, then $O(1/M_{P \ell})$ proton-decay amplitudes are
possible. These always conserve $B+L$, so that the dominant decay modes are $p
\rightarrow \Pi^+ \nu$ and $p \rightarrow \Pi^+ \Pi^+ \ell^-$, where $\Pi^+ =
\pi^+$ or $K^+$.
