solution could be understood in this framework. Moving away from minimal $SO(10)$, one may invoke a non-minimal Higgs sector in which one Higgs doublet arises from a ${\bf 10}$ and one from a ${\bf 16}$, and in this framework it is possible to understand atmospheric neutrino mixing [@AlBa]. Alternatively, one may invoke a non-minimal matter sector in which parts of a quark and lepton family arise from a ${\bf 16}$ and other parts from a ${\bf 10}$, and in these models one may account for atmospheric and solar neutrinos via an inverted mass hierarchy mechanism [@ShTa].

In the present paper we shall discuss neutrino masses and mixing angles in a particular string-inspired [*minimal*]{} model based on the Pati-Salam $SU(4)\times SU(2)_L \times SU(2)_R$ (422) group [@PaSa]. As in $SO(10)$ the presence of the gauged $SU(2)_R$ predicts the existence of three right-handed neutrinos. However, unlike $SO(10)$, there is no Higgs doublet-triplet splitting problem since in the minimal model both Higgs doublets are contained in a $(1,2,2)$ representation. Moreover, since the left-handed quarks and leptons are in the $(4,2,1)$ and the right-handed quarks and leptons in the $(4,1,2)$ representations, the model also leads to third family Yukawa unification as in minimal $SO(10)$. Although 