= Solution
Include both specific <antibodies> in the same <electrophoretic mobility shift assay> and compare with each single-antibody reaction and isotype controls. \b[An additional double supershift of one defined complex would support simultaneous occupancy], especially when reproduced with purified <TCF transcription factor>, <HNF1β> and one probe containing the paired sites. Mutating each site separately tests the relevant binding requirement. Failure to obtain a double supershift is not decisive because antibody binding may be sterically incompatible or disruptive. For the native promoter, sequential <chromatin immunoprecipitation> with the two factors, with reciprocal order and background controls, can test co-association with the same promoter-containing chromatin fragment; it does not alone prove direct contact or base-specific binding.
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