  We derive in a simple way the $(Z_2)^3$ symmetry which characterizes uniquely
the phenomenologically successful tripartite form leading to the tribimaximal
mixing in the neutrino mass matrix. We impose this symmetry in a setup
including the charged leptons and find that it can accommodate all the possible
patterns of lepton masses in the framework of type-I and type-II seesaw
mechanisms. We also discuss the possibility of generating enough baryon
asymmetry through lepton-lepton asymmetry.
