POWER OF ATTORNEY

KNOW ALL MEN BY THESE PRESENTS, that the undersigned hereby constitutes and
appoints CRAIG L. SLUTZKIN as his true and lawful attorney-in-fact and agent
for him and in his name, place and stead, in any and all capacities, to sign
any and all documents relating to any and all Securities and Exchange
Commission filings which may be required, granting unto said attorney-in-fact
and agent full power and authority to do and perform each and every act and
thing requisite and necessary to be done in and about the premises, as fully
to all intents and purposes as he might or could do in person, thereby
ratifying and confirming all that said attorney-in-fact may lawfully do or
cause to be done by virtue hereof.

This power shall continue in effect until terminated in writing.


By: /s/ Ronald Hunt
    -------------------------------------------------
      Ronald Hunt

Dated: September 29, 2006