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Fig. S1 No PBX-PREP-PDX1 complex is observed on homologous pax6a PA1 sequence. When we tested the homologous pax6a PA1 sequence (named PA1a), incubation with nuclear extract led to the formation of four major complexes (arrows, lane 1) which were displaced by adding a 100-fold molar excess of unlabeled PA1a (lane 2). Addition of unlabelled PA1b and somatostatin element UE-A, which bind PBX-PREP-PDX1 and PBX-PREP complex, respectively, do not displace the binding on the probe (lanes 3 and 4). This indicates that the proteins in the pancreatic extract which recognize the two cis-elements PA1b and UE-A do not recognize the homologous pax6a element.