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                                                                   Exhibit 10.25

                                  VIACELL (TM)

                                                        October 25, 2001

Genzyme Corporation
One Kendall Square
Cambridge, MA 02139
Attn:  Jan van Heek, Executive Vice President

Ladies and Gentlemen:

Reference is hereby made to the Series I Convertible Preferred Stock Purchase
Agreement (the "Purchase Agreement"), dated as of the date hereof, among
ViaCell, Inc. (the "Company") and the Purchasers listed therein. The capitalized
terms used herein and not otherwise defined herein shall have the meanings
assigned to such terms in the Purchase Agreement.

In connection with Genzyme Corporation's purchase of shares of the Company's
Series I Convertible Preferred Stock under the Purchase Agreement, the Company
hereby agrees that (i) the Company shall include in its proxy statement (the
"Proxy Statement") distributed in connection with each of its meetings of
stockholders (each a "Meeting") held for the purpose of electing directors to
the Company's Board of Directors (the "Board") within three (3) years after the
consummation of a Qualified Public Offering (as defined in the Company's Third
Amended and Restated Certificate of Incorporation) a nominee for director (a
"Nominee") who shall be designated by Genzyme and approved by the Board, which
approval shall not be unreasonably withheld or delayed, and (ii) the Board shall
recommend in such Proxy Statement for each such Meeting that the Company's
stockholders vote to elect such Nominee as a director of the Company; provided
however, that if directors of the Company are elected to serve for terms in
excess of one year pursuant to the Company's Certificate of Incorporation, the
obligation will relate to the minimum number of meetings required for Genzyme's
Nominee(s) to serve for three years (assuming the Nominee(s) is elected) after
the Qualified Public Offering. If, prior to the end of the three year period, a
vacancy is created on the Board due the death, removal or resignation of a
Nominee who was previously elected to the Board, the Board shall elect as a
director of the Company (an "Interim Director") a person who shall be designated
by Genzyme and approved by the Board, which approval shall not be unreasonably
withheld or delayed. For the avoidance of doubt, it is understood and agreed
that Genzyme shall not be entitled to have more than one Nominee serving on the
Board at any one time.

The Company shall deliver written notice of the date of each Meeting at least
one hundred and twenty (120 ) days in advance. Genzyme shall deliver notice to
the Company designating such Nominee (i) no later than ninety (90) days prior to
the date of the relevant Meeting or (ii) if the Company fails to notify Genzyme
of the date of the relevant Meeting at least one hundred and twenty (120) days
in advance, no later than thirty (30) days after Genzyme's receipt of such
notice from the Company. If such notice is not received by the Company in a
timely manner, the rights contained herein shall be deemed to be terminated with
respect to the relevant Meeting but shall continue in full force and effect for
any subsequent Meetings to which the rights described in the preceding paragraph
apply.

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Notwithstanding the foregoing, the Board shall not be required to take any
action that a majority of the directors determine in good faith is contrary to
the directors' fiduciary duties, as evidenced in a resolution adopted by the
Board, nor shall the Board or the Company be required to take any action that is
inconsistent with the requirements of applicable law, including the Delaware
General Corporation Law, or the Company's Certificate of Incorporation or the
Company's By-laws as then in effect.

In the event that the Board (i) does not approve of a Nominee or (ii) determines
that it cannot include such Nominee in a Proxy Statement or recommend that the
Company's stockholders elect such Nominee pursuant to the foregoing provisions
of this letter agreement, then the Company shall deliver written notice of such
determination within ten (10) business days after receipt of Genzyme's
designation notice and Genzyme's rights under this letter agreement shall apply
to an additional Meeting after the expiration of the aforementioned three year
period; provided, however, that in no event shall Genzyme have the right to
designate Nominees pursuant to this Agreement after the fifth (5th) anniversary
of the consummation of the Qualified Public Offering.

The Company hereby represents and warrants to Genzyme that as of the date hereof
the transactions contemplated hereby are not inconsistent with the Company's
Certificate of Incorporation or By-laws and agrees that until such time as
Genzyme's rights hereunder have expired, the Company will not take any action or
amend its Certificate of Incorporation or By-laws in a manner inconsistent with
or in derogation of this Agreement.

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If the foregoing represents our understanding, please countersign and return the
enclosed copy of this letter.

                                              Very truly yours,

                                              VIACELL, INC.


                                              By: /s/ Marc Beer
                                                 ------------------------------
                                                 Marc D. Beer
                                                  Chief Executive Officer

ACCEPTED AND AGREED:

GENZYME CORPORATION


By: /s/ Jan van Heck
    ----------------------------
Name:   Jan van Heck
Title:  Executive Vice President