The SV40 enhancer is composed of multiple functional elements that can compensate for one another.

We present evidence that the SV40 enhancer consists of three functional units, A, B, and C, each of which can cooperate with the others or with duplicates of itself to enhance transcription. We show that, when element C, containing the core consensus sequence, is inactivated by point mutations, revertants with restored enhancer function contain duplications of either one or both of the elements A and B. To search for additional elements, we isolated revertants of a mutant with the three elements mutated. These revertants do not identify any other elements; instead, enhancer function is effectively restored by "double duplications," in which the first duplication event either partially or entirely recreates one of the three elements A, B, and C and the second duplication then creates two copies of the newly created sequence.