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                                                                   EXHIBIT 4.5.1

                                                                  EXECUTION COPY

                     AMENDMENT TO FIFTH AMENDED AND RESTATED
                          REGISTRATION RIGHTS AGREEMENT


                                                 April 1, 2004



To each of the several holders of Restricted Stock named in Exhibit A attached
hereto.

Ladies and Gentlemen:

      Pursuant to the Registration Rights Agreement dated as of November 26,
1997 by and among t. Breeders, Inc., now known as Viacell, Inc., a Delaware
corporation (the "Company"), and the purchasers listed in the signature pages
thereto (as amended and restated by that certain Amended and Restated
Registration Rights Agreement, dated April 11, 2000, and as further amended by
the Second Amended and Restated Registration Rights Agreement, dated November
10, 2000, the Third Amended and Restated Registration Rights Agreement, dated
October 25, 2001, the Fourth Amended and Restated Registration Rights Agreement,
dated September 30, 2003 and the Fifth Amended and Restated Registration Rights
Agreement, dated December 23, 2003, the "Registration Rights Agreement"), the
Company granted certain registration rights to the holders of the Company's (i)
Series A Convertible Preferred Stock, $.01 par value, (ii) Series B Convertible
Preferred Stock, $.01 par value, (iii) Series C Convertible Preferred Stock,
$.01 par value, (iv) Series D Convertible Preferred Stock, $.01 par value, (v)
Series E Convertible Preferred Stock, $.01 par value, (vi) Series F Convertible
Preferred Stock, $.01 par value, (vii) Series G Convertible Preferred Stock,
$.01 par value, (viii) Series H Convertible Preferred Stock, $.01 par value,
(ix) Series I Convertible Preferred Stock, $.01 par value, (x) Series J
Convertible Preferred Stock, $.01 par value, (xi) Series K Convertible Preferred
Stock, $.01 par value, and (xii) the Warrant Agreements exercisable for Common
Stock (as defined herein) issuable pursuant to the Series D Unit/Series E
Preferred Stock Purchase Agreement, dated as of November 26, 1997, the Warrant
Agreements exercisable for Common Stock issuable to ARE-One, dated February 24,
2000, the Warrant Agreements exercisable for Common Stock issuable to Amgen
Inc., dated as of April 9, 2002, the Warrant Agreements exercisable for
Preferred Stock issuable to General Electric Capital Corporation, in connection
with the debt facility, dated October 16, 2003, and the Warrant Agreements
exercisable for Common Stock issuable pursuant to the Series J Purchase
Agreement, dated September 30, 2003. The parties to the Registration Rights
Agreement desire to amend the Registration Rights Agreement as set forth herein
(the "Amendment") in order to clarify certain provisions of the Registration
Rights Agreement. Capitalized terms used but not otherwise defined herein shall
have the meaning set forth in the Registration Rights Agreement.

      1. Amendment of Registration Rights Agreement. Section 13(g) of the
Registration Rights Agreement is hereby deleted in its entirety and replaced
with the following:

            (g) "Lock-Up" Agreement. If requested in writing by the underwriters
      for the initial underwritten public offering of securities of the Company,
      each holder of Restricted Stock who is a party to this Agreement shall
      agree not to sell publicly any shares of Restricted Stock or any other
      shares of Common Stock (other than shares of Restricted Stock or other
      shares of Common Stock being registered in such offering), without the
      consent of such underwriters, for a period of not more than 180 days
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      following the effective date of the registration statement relating to
      such offering; provided, however, that all persons entitled to
      registration rights with respect to shares of Common Stock who are not
      parties to this Agreement, all other persons selling shares of Common
      Stock in such offering, all persons holding in excess of 1% of the capital
      stock of the Company on a fully diluted basis and all executive officers
      and directors of the Company shall also have agreed not to sell publicly
      their Common Stock under the circumstances and pursuant to the terms set
      forth in this Section 13(g). Each holder of Restricted Stock further
      agrees to, and shall, execute any such agreement as may be requested by
      the underwriters in the Company's IPO that is consistent with this Section
      13(g) or that is necessary to give further effect thereto; provided that
      all such holders are requested by the underwriters to execute the same
      agreement. The underwriters in connection with the Company's IPO are
      intended third-party beneficiaries of this Section 13(g) and shall have
      the right, power and authority to enforce the provisions hereof as though
      they were a party hereto.

      2. Miscellaneous.

            (a) Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

            (b) Severability. If any provision of this Amendment shall be held
to be illegal, invalid or unenforceable, such illegality, invalidity or
unenforceability shall attach only to such provision and shall not in any manner
affect or render illegal, invalid or unenforceable any other provision of this
Amendment, and this Amendment shall be carried out as if any such illegal,
invalid or unenforceable provision were not contained herein.

            (c) Injunctive Relief. Each of the parties acknowledge that any
breach of this Amendment by any one of them will cause irreparable harm to the
other parties thereto and that in the event of such breach, the parties hereto
shall be entitled, in addition to monetary damages and to any other remedies
available under this Amendment and at law, to equitable relief, including
injunctive relief.

            (d) Ratification of Registration Rights Agreement. Except as
expressly amended hereby, the Registration Rights Agreement shall continue in
full force and effect, and the parties ratify the Registration Rights Agreement
in accordance with its terms.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

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      Please indicate your acceptance of the foregoing by signing and returning
the enclosed counterpart of this letter, whereupon this Amendment to the Rights
Agreement shall be a binding agreement between the Company and you.

                                        VIACELL, INC.




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


AGREED TO AND ACCEPTED as of the date first above written.

                      [PURCHASER SIGNATURE PAGE TO FOLLOW]

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                                                                  EXECUTION COPY

                                  VIACELL, INC.

                     SIGNATURE PAGE FOR HOLDERS OF WARRANTS,
               SERIES A PREFERRED STOCK, SERIES B PREFERRED STOCK,
               SERIES C PREFERRED STOCK, SERIES D PREFERRED STOCK,
               SERIES E PREFERRED STOCK, SERIES F PREFERRED STOCK,
               SERIES G PREFERRED STOCK, SERIES H PREFERRED STOCK,
               SERIES I PREFERRED STOCK, SERIES J PREFERRED STOCK
                          AND SERIES K PREFERRED STOCK,

By his, her or its execution and delivery of this signature page, the
undersigned hereby joins in and agrees to be bound by the terms and conditions
of each of the Amendment to Fifth Amended and Restated Registration Rights
Agreement, subject to, and as of the date of, the execution and delivery of such
Amendment by ViaCell, Inc., and authorizes this signature page to be attached to
the Amendment to Fifth Amended and Restated Registration Rights Agreement,
Agreement, or counterparts thereof.

           EXECUTED this ____ day of _________, 2004.


                                        ____________________________________
                                        Print Name of Investor

                                        By: ________________________________
                                        Name: ______________________________
                                        Title: _____________________________

                                        Record Address: ____________________
                                        ____________________________________
                                        ____________________________________

                                        Facsimile No.: _____________________