Agashe, Contino, Da Rold, and Pomarol recently realized that assigning top and bottom quarks to different representations of SU(2)_L x SU(2)_R can protect the coupling of the Z to the left-handed bottom quark from non-standard model corrections. Typically extra dimensional models of electroweak symmetry breaking with low mass scales for the Kaluza-Klein modes have corrections of the order of 20-40% while measurements from

LEP and SLAC require less than a 0.25% deviation from the standard model prediction. The left-handed top and bottom quarks in their models

are in the (2,2) representation rather than the (2,1) so there is a

heavy top and an exotic quark to look for.