Supersymmetry Breaking Threshold Corrections in the SU(4)times SU(2)_Ltimes SU(2)_R Model

We evaluate the SUSY and top threshold effects in the context of the MSSM and the string derived model based on SU(4)$\times$SU(2)$_L\times$SU(2)$_R$. In both cases we run the two loop RGEs and determine the lower bounds of the supersymmetric particle masses, dictated by the experimentally accepted regions of the values of the low energy parameters.