ime limits. An important benefit of the SU421 symmetry breaking pattern is that these colour triplets may be projected out by the Generalised GSO (GGSO) projections [@ps], and need not be present in the low energy spectrum. The string doublet–triplet mechanism works in all models that include the symmetry breaking pattern $SO(10)\rightarrow SO(6)\times SO(4)$. The HSPSM heavy Higgs states, which break $SU(4)\times SU(2)_R\rightarrow SU(3)_C\times U(1)_{Y}$, contain colour triplets with the charges of the states in (\[SU421dr\]) that may give rise to dimension five proton decay mediating operators. In the HSPSM the superpotential terms $\lambda_2HHD+\lambda_3 {\bar H}{\bar H}{\bar D}$ couples the colour triplets from the vectorial representation $(6,1,1)$ to the colour triplets arising from the heavy Higgs field. The GUT scale VEVs of the heavy Higgs fields $H$ and ${\bar H}$ are used to give heavy mass to the Higgs colour triplets. However, the heavy Higgs representations in the SU421 heterotic–string models, eq. (\[SU421Higgs\]), do not contain the states with the charges of eq. (\[SU421dr\]). Consequently, the stringy doublet–triplet splitting mechanism works only in models in which the $SO(10)$ symmetry is broken to $SU(3)_C\times SU(2)_L\times U(1)^2$, $SU(4)_C\times SU(2)_L\times U(1)_R$, or $SU(3)_C\times SU(2)_L\times SU(2)_R \times U(1)_{B-L}$.

Another important advantage of the SU421 class of models versus the PS and LRS models is with respect to the light Higgs representations. In the LRS and PS models, the light Higgs states exist in bi–doublet representations and couple simultaneously to the up– and down–type quarks, which may give rise to Flavor Changing Neutral Currents (FCNC) at an unacceptable rate [@lrsfcnc]. This introduces a bi–doublet splitting problem. The solutions that have been proposed in the literature [@bidoub] use a $SU(2)_L$ triplet representation that are not present in string models in which the gauge symmetry is realised as a level one Kac–Moody algebra. On the oth