  Precision measurements at the $Z$ resonance agree well with the standard
model. However, there is still a hint of a discrepancy, not so much in $R_b$ by
itself (which has received a great deal of attention in the past several years)
but in the forward-backward asymmetry $A^b_{FB}$ together with $R_b$. The two
are of course correlated. We explore the possibilty that these and other
effects are due to the mixing of $b_L$ and $b_R$ with one or more heavy quarks.
