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                                                                     Exhibit 4.9


                               THIRD AMENDMENT TO
             STANDARD INDUSTRIAL/COMMERCIAL SINGLE-TENANT LEASE-NET

         THIS THIRD AMENDMENT TO STANDARD INDUSTRIAL/COMMERCIAL SINGLE-TENANT
LEASE-NET (this "Amendment") is made and entered into as of October 4, 2001, by
and between HASERJIAN BROS. REALTY CO., a California general partnership
("Lessor"), and QIAGEN, INC. ("Lessee").

                                    RECITALS:

         A. Lessor and Lessee are parties to a certain Standard
Industrial/Commercial Single- Tenant Lease-Net dated May 15, 1996, as amended by
that certain First Amendment dated February 10, 1999 and that certain Second
Amendment dated May 16, 2001 ("Second Amendment") (collectively, the "Original
Lease") relating to the "Premises," as more particularly described therein.
Unless otherwise defined herein, all capitalized terms used herein shall have
the meanings set forth in the Original Lease.

         B. Lessor and Lessee desire to amend the Original Lease as set forth in
this Amendment (the Original Lease, as amended by this Amendment shall be
referred to hereinafter as the "Lease").

         NOW, THEREFORE, for and in consideration of the foregoing recitals and
the mutual covenants and agreements contained herein, Lessee and Lessor agree as
follows:

         1. Amendment of Paragraph 56. Lessee agrees that prior to the
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expiration or earlier termination of the Lease, Lessor has the right to require
Lessee to remove, on the expiration or earlier termination of the Lease, any or
all of the leasehold improvements described in Paragraph 56 (LESSEE'S
IMPROVEMENTS) of the Second Amendment.

         2. Effect of this Amendment. Except where conflicting or inconsistent
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with the express terms or manifest intent of this Amendment, all provisions of
the Original Lease as in effect prior to this Amendment shall remain in full
force and effect. Wherever there is a conflict or inconsistency between any of
the provisions in this Amendment and the Original Lease, the provisions of this
Amendment shall be deemed to govern and control. No party to this Amendment
shall be deemed to waive any of the rights or remedies available to it under the
Lease, all of which are expressly reserved.

         3. Miscellaneous.
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            (a)   Governing Law. This Amendment shall be governed by and
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construed in accordance with the laws of the State of California applicable to
agreements negotiated executed and performed in California.

            (b)   Progressive Integration. The Original Lease, as modified only
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by the Amendment, together with the exhibits annexed thereto and hereto,
contains all of the agreements of Lessee and Lessor with regard to the subject
transactions, and supersedes all negotiations, discussions, understandings and
agreements, whether written or oral, existing prior to the date of this
Amendment.

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            (c)   Execution in Counterparts and by Facsimile. This Amendment may
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be executed in any number of counterparts, each of which when so executed and
delivered shall be deemed to be an original and all of which when taken together
shall constitute one and the same agreement. A facsimile execution copy of this
Amendment shall be binding and have the same force and effect as the original of
this Amendment.

         IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the date first written above.

                              LESSOR:        HASERJIAN BROS. REALTY CO.,
                                             a California general partnership

                                             By: /s/ Ted Haserjian
                                                 -----------------
                                             Name: Ted Haserjian
                                             Its: General Partner

                                             By: /s/ Harold Haserjian
                                                 --------------------
                                             Name: Harold Haserjian
                                             Its: General Partner



                              LESSEE:        QIAGEN, INC.



                                             By: /s/ Rosalie Duong
                                                 -----------------
                                             Name: Rosalie Duong
                                             Its: President

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