  As pointed out by many authors, recent observations are consistent with an
intriguing relation between the Cabibbo angle $\theta_C$ and the solar neutrino
mixing angle $\theta_{12}$, namely $\theta_{12} \simeq \pi/4 - \theta_C$. Such
quark-lepton complementarity (QLC) may be a signal of an underlying
quark-lepton unification at short distances. We discuss possible ways to
realize this relation in realistic quark-lepton unification theories by
identifying a minimal set of operators that lead to QLC while remaining
consistent with other known data. The purpose of this paper is to present the
first elements of a unified model at the GUT scale capable of predicting the
QLC relation. A generic prediction of our proposed class of models is the new
relation for the lepton mixing angle $\theta_{13} \simeq \theta_C$, which
allows these models to be confirmed or excluded by the current generation of
neutrino oscillation experiments.
