Functional and cooperative interactions between the homeodomain PDX 1 , Pbx , and Prep 1 factors on the somatostatin promoter . ^^^ In contrast , in the context of the intact somatostatin promoter or mini enhancer , Pbx 1 and Prep 1 alone have no effect , but they produce a drastic activation when the pancreatic homeodomain factor PDX 1 is also coexpressed . ^^^