Tissue regeneration enhancer elements (TREEs) control regeneration capacity in zebrafish. (a) After amputation injuries, a TREE linked to the leptin b gene (LEN) can rescue regeneration defects in adult fgf20a mutants (dob) when paired with an fgf20a expression cassette. (b) Conversely, the same TREE can effect a block in tailfin regeneration in the wild-type background when paired with the expression of a dominant negative Fgfr1. Intriguingly, these transgenic animals undergo normal development, whereas their regeneration capacity is modified. Thus, the LEN element appears to be specifically activated after injury and during regeneration.