Seesaw scales and the steps from the Standard Model towards   superstring-inspired flipped E_6
  Recently in connection with Superstring theory E_8 and E_6 unifications became very promising. In the present paper we have investigated a number of available paths from the Standard Model (SM) to the E_6 unification, considering a chain of flipped models following the extension of the SM:   SU(3)_C\times SU(2)_L\times U(1)_Y \to   SU(3)_C\times SU(2)_L\times U(1)_X \times U(1)_Z \to   SU(5)\times U(1)_X \to   SU(5)\times U(1)_{Z1} \times U(1)_{X1} \to   SO(10) \times U(1)_{X1} \to   SO(10) \times U(1)_{Z2}\times U(1)_{X2} \to   E_6\times U(1)_{X2} or E_6,   Also we have considered a chain with a left-right symmetry:   SU(3)_C\times SU(2)_L\times U(1)_Y \to   SU(3)_C\times SU(2)_L \times SU(2)_R\times U(1)_X\times U(1)_Z \to   SU(4)_C\times SU(2)_L \times SU(2)_R\times U(1)_Z \to   SO(10)\times U(1)_Z \to   E_6.   We have presented four examples including non-supersymmetric and supersymmetric extensions of the SM and different contents of the Higgs bosons providing the breaking of the flipped SO(10) and SU(5) down to the SM. It was shown that the final unification E_6\times U(1) or E_6 at the (Planck) GUT scale M_{SSG} depends on the number of the Higgs boson representations considered in theory. 
C.R. Das, Larisa Laperashvili