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                                                                   EXHIBIT 10.21
                             AMENDMENT #6 TO LEASE
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1.   Parties.
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     This Amendment, dated as of April 17, 2001, is between Andover Mills Realty
Limited Partnership ("Landlord") and CMGI, Inc. ("Tenant").

2.   Recitals.
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     2.1  Landlord and Tenant have entered into Lease, dated as of April 12,
1999, for space in Brickstone Square in Andover, Massachusetts (as now or
hereafter amended, the "Lease").  Unless otherwise defined, terms used in this
Amendment have the same meanings as those used in the Lease.

     2.2  Tenant wishes to grant to Landlord the right and option to terminate
the Lease with respect to Offer Space #2, consisting of 9,472 s.f. of agreed
rentable area on the 2nd Floor of Building 200.  The parties hope that Landlord
will be able to lease Offer Space #2 to others and then exercise this
termination option, although Landlord is not obligated either to lease or
exercise. Subject to the terms of this Amendment, if Landlord validly exercises
its option, Tenant will assign and transfer its interest in Offer Space #2 to
Landlord and the Lease will then terminate with respect to Offer Space #2 only.
In order to accomplish this and other matters, for $10.00 and other good and
valuable consideration, the receipt and sufficiency of which is acknowledged,
the parties agree and the Lease is amended as follows as of the date hereof,
notwithstanding anything to the contrary:

3.   Amendments.
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     3.1  Tenant hereby grants to Landlord the right and option to terminate the
Lease with respect to Offer Space #2 only.  Landlord has no obligation to
exercise this option, but if it elects to exercise it will do so by delivering a
written exercise notice to Tenant.

     3.2  As of the date that Landlord validly exercises this option, Tenant
will be deemed to have assigned, conveyed and transferred to Landlord all of
Tenant's right, title and interest in and to Offer Space #2, and: (a) the Lease
will terminate and expire as to Offer Space #2 only and Tenant and its
Affiliates will vacate and surrender possession of Offer Space #2; (b) the
agreed rentable area of the remainder of the Premises will be reduced by 9,472
s.f.; and (c) Tenant's parking rights under the Lease will be reduced by twenty-
eight (28) vehicles, Exhibit "A-5" attached to Amendment #5 to Lease will be
deemed deleted from the Lease and Exhibit "A-7" attached hereto and incorporated
herein (which shows a new parking plan for Tenant) will be substituted in its
place.  Despite the earlier termination of the Lease with respect to Offer Space
#2, and in addition to other rent payable under the Lease, all rent that would
have been payable by Tenant in connection with Offer Space #2 absent this
termination will continue to be paid as rent under the Lease through the date
(the "Initial Rent Termination Date") that is the first to occur of: ninety (90)
days after Landlord exercises this option; or the date that a new tenant under a
new lease of Offer Space #2 occupies Offer Space #2 to conduct business; or the
rent commencement date under any new lease of Offer Space #2.  After the Initial
Rent Termination Date, rent will terminate  entirely for 5,000 s.f. of Offer
Space #2, and for the next one (1) calendar year after the Initial Rent
Termination Date, Tenant will continue to pay only the rent that would have been
payable by Tenant in connection with Offer Space #2 absent this

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termination for the remainder of Offer Space #2 (i.e., on 4,472 s.f). Tenant
will not be required to pay any rent in connection with Offer Space #2 for any
period after this one (1)-year period. (As a hypothetical example, if Landlord
exercises this option and leases Offer Space #2 to a new tenant under a new
lease with a rent commencement date thirty (30) days after exercise, and no
earlier occupation by the new tenant, rent payable by Tenant in connection with
Offer Space #2 would continue for all of that space until thirty (30) days after
exercise [i.e., until the Initial Rent Termination Date], and it would continue
for only 4,472 s.f. of that space until one (1) year after that Initial Rent
Termination Date, after which it would terminate entirely.) In Section 27(e)(i)
of the Lease, the figure for the area set forth therein will be reduced by 9,472
s.f. Following Landlord's exercise of this option with respect to Offer Space
#2, neither party will have any further rights or obligations to the other with
respect thereto (except for any unpaid rent due under the Lease or hereunder for
any period before the rent is to terminate with respect to that portion of Offer
Space #2).

     3.3  Following the Rent Termination Date of the Lease with respect to all
of Offer Space #2, Tenant's Percentage will be reduced by 1.01%, to a total of
33.14%, assuming that Tenant leases no other space in the Project.

     3.4  As a material inducement to Landlord to enter into this Amendment,
Tenant agrees that, as of the date hereof, Landlord is not, to Tenant's
knowledge, in default under the Lease, and Tenant represents to Landlord that
Tenant has not subleased, assigned or conveyed Offer Space #2 or its interests
therein to anyone else.

     3.5  As a material inducement to Tenant to enter into this Amendment,
Landlord agrees that, as of the date hereof, Tenant is not, to Landlord's
knowledge, in default under the Lease.

     Time is of the essence in this Amendment and holding over will not be
permitted.  The Lease remains in full force and effect, and except as set forth
above, the Lease remains unchanged.

     IN WITNESS WHEREOF, intending to be legally bound, the parties have
executed this Amendment as of the date in Article 1 above.

CMGI, Inc., a Delaware corporation    Andover Mills Realty Limited
                                      Partnership, a Massachusetts limited
                                      partnership
By:  /s/ Andrew J. Hajducky III
     Name: Andrew J. Hajducky III     By:  Brickstone Square Realty, Inc., a
     Title: Chief Financial Officer        Massachusetts corporation, general
     Authorized Signature                  partner

                                           By:  /s/ Martin Spagat
                                                Name: Martin Spagat
                                                Title: Vice President
                                                Authorized Signature

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