We impose new limits on flavor changing neutral currents (fcnc) applied to the 4×4 mixing matrix of the next heaviest down quark in a model with extraSU(2)Lsinglet down quarks, such as inE6. We find the new bounds still allow tree levelZexchange to dominateBd0−B¯d0mixing, but notBs0−B¯s0mixing. The unitarity ‘‘triangle’’ may still have added a detectable fourth side. TheCPasymmetries fromBd0may show a newCPviolating phase or the usual CKM phase.