Finally, there are a few examples of proteins that undergo different structural transitions as a function of the partner they bind. Notably, HIFa (hypoxia-inducible factor a ) possesses an intrinsically disordered domain, which can interact with two different partners. This unstructured domain adopts an a-helical structure when bound to the TAZ1 domain of CBP. However, binding of the same region of HIFa to an asparagine hydroxylase results in a highly extended conformation, which is required for the enzymatic activity of the latter. These findings provide an elegant example of the plasticity that intrinsically disordered proteins display for different partners involved in different functions. We can speculate on a similar behavior in the case of N TAIL . In particular, Box2 and Box3 of N TAIL interact both with at least two distinct partners, P and Hsp72, and competition between XD and Hsp72 for binding to N TAIL has been recently shown (Zhang et al. 2005) . It is conceivable that binding of N TAIL to P or to Hsp72 may involve a different structural transition, as in the case of HIFa.