Explanation of Responses:

      (1)   In addition to Baker Brothers Life Sciences Capital (GP), LLC
            (formerly known as Baker Biotech Capital III (GP), LLC), this form
            is being filed jointly by Julian C. Baker, Felix J. Baker, Baker
            Biotech Capital II (GP), LLC and Baker Biotech Capital II (Z)(GP),
            LLC, each of whom has the same business address as Baker Brothers
            Life Sciences Capital (GP), LLC. Each of the reporting persons
            disclaims beneficial ownership of the securities reported hereon
            except to the extent of his or its pecuniary interest, if any,
            therein.

      (2)   On April 25, 2006, Baker Biotech Fund II, L.P., Baker Biotech Fund
            II (Z), L.P. and Baker Biotech Fund III (Z), L.P. were merged with
            and into Baker Biotech Fund III, L.P. which changed its name
            simultaneously therewith to Baker Brothers Life Sciences, L.P.
            Simultaneously therewith, Baker Biotech Capital II, L.P., Baker
            Biotech Capital II (Z), L.P., and Baker Biotech Capital III (Z),
            L.P., the general partners of Baker Biotech Fund II, L.P., Baker
            Biotech Fund II (Z), L.P. and Baker Biotech Fund III (Z), L.P.,
            respectively, were merged with and into Baker Biotech Capital III,
            L.P., the general partner of Baker Biotech Fund III, L.P., and Baker
            Biotech Capital III, L.P. changed its name simultaneously therewith
            to Baker Brothers Life Sciences Capital, L.P. In addition,
            simultaneously therewith, Baker Biotech Capital II (GP), LLC, Baker
            Biotech Capital II (Z)(GP), LLC, and Baker Biotech Capital III
            (Z)(GP), LLC, the general partners of Baker Biotech Capital II,
            L.P., Baker Biotech Capital II (Z), L.P., and Baker Biotech Capital
            III (Z), L.P., respectively, were merged with and into Baker Biotech
            Capital III (GP), LLC, the general partner of Baker Biotech Capital
            III, L.P. and Baker Biotech Capital III (GP), LLC changed its name
            simultaneously therewith to Baker Brothers Life Sciences Capital
            (GP), LLC. Julian C. Baker and Felix J. Baker are the controlling
            members of Baker Biotech Capital II (GP), LLC, Baker Biotech Capital
            II (Z)(GP), LLC, Baker Biotech Capital III (Z)(GP), LLC and Baker
            Brothers Life Sciences Capital (GP), LLC, and may have a pecuniary
            interest in shares held by them. Such pecuniary interest, if any,
            was unchanged as a result of the mergers referred to herein. Julian
            C. Baker and Felix J. Baker disclaim beneficial ownership of the
            shares reported herein except to the extent of their pecuniary
            interest, if any, therein.

      (3)   Represents shares owned directly by Baker Biotech Fund II, L.P.,
            which is no longer subject to Section 16 of the Securities Exchange
            Act of 1934.

      (4)   Represents shares owned directly by Baker Biotech Fund II (Z), L.P.,
            which is no longer subject to Section 16 of the Securities Exchange
            Act of 1934.

      (5)   Represents shares owned directly by Baker Brothers Life Sciences,
            L.P. (formerly known as Baker Biotech Fund III, L.P.).