  Neutrino mixing in the standard model extensions, both renormalizable and
effective, with arbitrary numbers of the singlet and left-handed doublet
neutrinos is investigated in a systematic fashion. The charged and neutral (the
Z and Higgs mediated)lepton currents are written under general Majorana
condition, and the odservable independence of the choice of the condition, the
rephasing invariance, is studied. A parametrization of the neutrino mixing
matrices in the doublet-singlet factorized form is developed. Its relationship
with the see-saw mechanism is shown in the limit of a small doublet-singlet
mixing. The structure of the mixing matrices relevant to the neutrino
oscillation experiments is explicated.
