Novel Relations between Lepton and Quark Mixings

A novel relation is found which gives the 2-3 lepton mixing angle in terms of quark masses and CKM mixing: \tan\theta_{\mu\tau}=(m_b/m_s)V_{cb}. This relation, which is remarkably in good agreement with the quark data and the large lepton mixing recently observed, is a kind of SO(10) GUT relation similar to the celebrated bottom-tau mass ratio in SU(5). The GUT models in which this relation hold should have a `twisted SO(10)' structure between the second and third generations, in order to really explain the large lepton mixing, or equivalently, the right-hand side of the relation.