Constraints on natural Maki-Nakagawa-Sakata parameters from |U _e3|

The MNS matrix structure emerging as a result of recent neutrino measurements strongly suggests two large mixing angles (solar and atmospheric) and one small angle (|U_e3|≪1). Especially when combined with the neutrino mass hierarchy, these values turn out to impose rather stringent constraints on possible flavor models connecting the three active fermion generations. Specifically, we show that an extremely small value of |U _e3| would require fine-tuning of Majorana mass matrix parameters, particularly in the context of seesaw models.