  Enhanced radiative corrections generated in SUSY extensions of the Standard
Model make the fit of the precision data ($Z$-boson decay parameters and
$W$-boson mass) worse. This negative effect is washed out for heavy enough
squarks due to decoupling property of SUSY models. We find that even for light
squarks the enhanced radiative corrections can be small. In this case
substantial $\tilde{t}_L \tilde{t}_R$ mixing is necessary.
