Practice question

In newt forelimb regeneration retinoic acid typically proximalizes positional identity reprogramming distal blastema toward proximal fate. Applied topically or via implanted beads, RA activates RAR-dependent transcription of Meis1/2 and proximal Hox9 genes, increases Prod1 expression level characteristic of proximal cells, suppresses distal HoxA13. Resulting regenerate duplicates proximal structures, converting wrist blastema into complete limb. Endogenous gradient with high RA proximally shapes normal pattern. Dose-dependent transformations demonstrated by Maden confirm proximalizing action; thus RA functions as respecifying morphogen resetting positional memory rather than eliminating identity or having no effect.