Expression of Ubx^{Ia.Δ20-61.Scer\UAS.hs.T:Ivir\HA1}, under the control of Scer\GAL4^arm.PS, at the same levels as endogenous Ubx results in an 88% repression of larval limbs. However, at half the normal concentration of Ubx protein, Ubx^{Ia.Δ20-61.Scer\UAS.hs.T:Ivir\HA1} represses only 11% of larval limbs, sixfold less than the equivalent concentration of wild-type Ubx protein. A double mutant, expressing Ubx^{Ia.Δ20-61.Scer\UAS.hs.T:Ivir\HA1} and Ubx^{Ia.Δ234-251.Scer\UAS.hs.T:Ivir\HA1}, under the control of Scer\GAL4^arm.PS, exhibits an additive effect in limb repression capacity, repressing about 50% of larval limbs.