Exhibit 10.34

FOURTH
AMENDMENT TO LEASE

THIS
FOURTH AMENDMENT TO LEASE (this "Fourth Amendment") is made as of the 25th day of October, 2007 (the "Fourth Amendment Date"), by and between THE REALTY ASSOCIATES FUND
VI, L.P., a Delaware limited partnership ("Landlord") and CUBIST PHARMACEUTICALS, INC., a Delaware corporation ("Tenant").

RECITALS:

WHEREAS,
by a lease (the "Original Lease") dated as of January, 2004, California State Teachers' Retirement System ("Calsters") leased to Tenant approximately 15,475 rentable square feet
of space, consisting of a portion of the second (Suite 2201-55) and fourth (Suite 4201-55) floors in the building known as 45-55 Hayden Avenue,
Lexington, Massachusetts (the "Building"); and

WHEREAS,
Landlord has succeeded to the interests of Calsters as landlord under the Lease; and

WHEREAS,
the Original Lease has been amended by a First Amendment to Lease between Landlord and Tenant, dated as of September 29, 2005 (the "First Amendment"), and by a Second
Amendment to Lease between Landlord and Tenant dated as of November 18, 2005 (the "Second Amendment"), and by a Third Amendment to Lease between Landlord and Tenant dated as of June 20,
2007 (the "Third Amendment"), (the Original Lease, as so amended, being referred to as the "Lease"), pursuant to which the size of the premises demised under the Original Lease was increased to 83,097
rentable square feet (the "Existing Premises"). The Existing Premises consist of Suites 2201-55, 2200-55, 3000-55, 4201-55 and 4200-55; and

WHEREAS,
by letter dated as of April 27, 2006, and pursuant to the Second Amendment, Tenant elected to include Suite 2200-55 (also known as the Comet Space) in
the Existing Premises for the remainder of the Lease Term; and

WHEREAS,
Landlord and Tenant now desire to further amend the Lease to, among other things, expand the size of the Existing Premises by adding thereto approximately 25,405 rentable square
feet of space in Suite 4600-45 on the fourth floor of the Building (the "Additional Fourth Floor Space"), and to adjust the rent and certain provisions, all on the terms and
conditions set forth below.

NOW,
THEREFORE, in consideration of the mutual covenants herein contained, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged,
and intending to be legally bound hereby, Landlord and Tenant hereby agree as follows:



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PREMISES:

The following areas in the 55 Hayden Avenue portion of the Building: (i) Suite 2201-55 on the second floor and Suite 4201-55 on the fourth floor, (ii) Suite 4200-55 on the fourth floor of the
Building, (iii) Suite 2200-55 on the second floor of the Building, and (iv) Suite 3000-55 on the third floor of the Building; and Suite 4600-45 on the fourth floor of the 45 Hayden Avenue portion of the Building. All such
spaces are shown onExhibit A-4,attached hereto and incorporated herein.RENTABLE FLOOR AREA OF THE PREMISES:

Approximately 108,502 square feet of the 55 Hayden Avenue portion of the Building as follows (i) 6,755 rentable square feet on the second floor contained in Suite 2201-55, (ii) 8,720 rentable square feet on the fourth floor contained
in Suite 4201-55 and 31,453 rentable square feet on the fourth floor of the Building contained in Suite 4200-55, (iii) 6,150 rentable square feet on the second floor of the Building contained in Suite 2200-55, and (iv) 30,019
rentable square feet on the third floor of the Building contained in Suite 3000-55; and 25,405 rentable square feet on the fourth floor of the 45 Hayden Avenue portion of the Building contained in Suite 4600-45;.




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IN
WITNESS WHEREOF, the undersigned have hereunto set their hands and seals as of the date first above written.

| | LANDLORD:                                                          
| | -------------------------------------------------------------------
| | The Realty Associates Fund VI, L.P.,
a Delaware limited partnership
| | By:                                                                
| |                                                                    
| |                                                                    
| |                                                                    
| | By:                                                                
| | By:                                                                
| | TENANT:                                                            
| | CUBIST PHARMACEUTICALS, INC.                                       
| | By:                                                                
| |                                                                    

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Exhibit A-4

Plans
of Premises

[Floor
Plans]

|SECOND FLOOR
45-55 HAYDEN AVENUE
LEXINGTON, MA |  | CUBIST SUITE 2200
6,150 RSF
|---------------------------------------------- |  | ---------------------------

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[Floor
Plans]

|SECOND FLOOR
45-55 HAYDEN AVENUE
LEXINGTON, MA |  | CUBIST SUITE 2201
6,755 RSF
|---------------------------------------------- |  | ---------------------------

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[Floor
Plans]

|THIRD FLOOR
45-55 HAYDEN AVENUE
LEXINGTON, MA |  | MOTOROLA SUITE 3000
30,019 RSF
|--------------------------------------------- |  | ------------------------------

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[Floor
Plans]

|FOURTH FLOOR
45-55 HAYDEN AVENUE
LEXINGTON, MA |  | CUBIST SUITE 4200
31,453 RSF
|---------------------------------------------- |  | ----------------------------

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[Floor
Plans]

|FOURTH FLOOR
45-55 HAYDEN AVENUE
LEXINGTON, MA |  | CUBIST SUITE 4201
8,720 RSF
|---------------------------------------------- |  | ---------------------------

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[Floor
Plans]

|FOURTH FLOOR
45 HAYDEN AVENUE
LEXINGTON, MA |  |           
|------------------------------------------- |  | ----------
|CUBIST SUITE 4600-45                        |  | 25,405 RSF

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SCHEDULE
I TO FOURTH AMENDMENT
Annual Rent

Annual
Rent for the Premises shall be the sum of the rents shown below for each portion of the Premises, determined as of the date in question.

As to Suites 2201-55, 4201-55 and 4200-55:

|RENTAL PERIOD  |  | TOTAL ANNUAL
RENT |            | MONTHLY PAYMENT |   | RENTAL RATE/SF
|-------------- |  | ----------------- | ---------- | --------------- | - | --------------
|5/1/061/31/07 |  | $                 | 371,400.00 |                 | $ | 30,950.00     
|2/1/071/31/08 |  | $                 | 386,875.00 |                 | $ | 32,238.58     
|2/1/087/31/09 |  | $                 | 402,350.00 |                 | $ | 33,529.17     

plus
(ii) as to Suite 4200-55:

|RENTAL PERIOD  |  | TOTAL ANNUAL
RENT |            | MONTHLY PAYMENT |   | RENTAL RATE/SF
|-------------- |  | ----------------- | ---------- | --------------- | - | --------------
|7/1/064/30/07 |  | $                 | 723,419.00 |                 | $ | 60,284.92     
|5/1/074/30/08 |  | $                 | 739,145.50 |                 | $ | 61,595.46     
|5/1/084/30/09 |  | $                 | 754,872.00 |                 | $ | 62,906.00     
|5/1/097/31/09 |  | $                 | 794,188.25 |                 | $ | 66,182.35     

|RENTAL PERIOD  |  | TOTAL ANNUAL
RENT |              | MONTHLY PAYMENT |   | RENTAL RATE/SF
|-------------- |  | ----------------- | ------------ | --------------- | - | --------------
|8/1/094/30/10 |  | $                 | 1,184,932.00 |                 | $ | 98,744.33     
|5/1/104/30/11 |  | $                 | 1,208,396.00 |                 | $ | 100,699.67    
|5/1/114/30/12 |  | $                 | 1,231,860.00 |                 | $ | 102,655.00    
|5/1/124/30/13 |  | $                 | 1,267,056.00 |                 | $ | 105,588.00    
|5/1/134/30/14 |  | $                 | 1,325,716.00 |                 | $ | 110,476.33    
|5/1/144/30/15 |  | $                 | 1,384,376.00 |                 | $ | 115,364.67    
|5/1/154/30/16 |  | $                 | 1,419,572.00 |                 | $ | 118,297.67    

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As to Suite 2200-55:

|RENTAL PERIOD  |  | TOTAL ANNUAL
RENT |            | MONTHLY PAYMENT |   | RENTAL RATE/SF
|-------------- |  | ----------------- | ---------- | --------------- | - | --------------
|6/1/064/30/07 |  | $                 | 140,415.00 |                 | $ | 11,787.50     
|5/1/074/30/08 |  | $                 | 144,525.00 |                 | $ | 12,043.75     
|5/1/084/30/09 |  | $                 | 147,600.00 |                 | $ | 12,300.00     
|5/1/094/30/10 |  | $                 | 155,287.50 |                 | $ | 12,940.63     
|5/1/104/30/11 |  | $                 | 158,362.50 |                 | $ | 13,196.88     
|5/1/114/30/12 |  | $                 | 161,437.50 |                 | $ | 13,453.13     
|5/1/124/30/13 |  | $                 | 166,050.00 |                 | $ | 13,837.50     
|5/1/134/30/14 |  | $                 | 173,737.50 |                 | $ | 14,478.13     
|5/1/144/30/15 |  | $                 | 181,425.00 |                 | $ | 15,118.75     
|5/1/154/30/16 |  | $                 | 186,037.50 |                 | $ | 15,503.13     

As to Suite 3000-55:

|RENTAL PERIOD   |  | TOTAL ANNUAL
RENT |              | MONTHLY PAYMENT |   | RENTAL RATE/SF
|--------------- |  | ----------------- | ------------ | --------------- | - | --------------
|Through 4/30/08 |  | $                 | 780,494.00   |                 | $ | 65,041.17     
|5/1/084/30/09  |  | $                 | 810,513.00   |                 | $ | 67,542.75     
|5/1/094/30/10  |  | $                 | 825,522.50   |                 | $ | 68,793.54     
|5/1/104/30/11  |  | $                 | 840,532.00   |                 | $ | 70,044.33     
|5/1/114/30/12  |  | $                 | 900,570.00   |                 | $ | 75,047.50     
|5/1/124/30/13  |  | $                 | 945,598.50   |                 | $ | 78,799.88     
|5/1/134/30/14  |  | $                 | 975,617.50   |                 | $ | 81,301.46     
|5/1/144/30/15  |  | $                 | 1,035,655.50 |                 | $ | 83,304.63     
|5/1/154/30/16  |  | $                 | 1,065,674.50 |                 | $ | 88,806.21     

As to Suite 4600-45

|RENTAL PERIOD   |  | TOTAL ANNUAL
RENT |            | MONTHLY PAYMENT |   | RENTAL RATE/SF
|--------------- |  | ----------------- | ---------- | --------------- | - | --------------
|Through 4/30/09 |  | $                 | 819,311.25 |                 | $ | 68,275.94     
|5/1/094/30/10  |  | $                 | 844,716.25 |                 | $ | 70,393.02     
|5/1/104/30/11  |  | $                 | 870,121.25 |                 | $ | 72,510.10     
|5/1/114/30/12  |  | $                 | 895,526.25 |                 | $ | 74,627.19     
|5/1/124/30/13  |  | $                 | 920,931.25 |                 | $ | 76,744.27     
|5/1/134/30/14  |  | $                 | 946,336.25 |                 | $ | 78,861.35     
|5/1/144/30/15  |  | $                 | 971,741.25 |                 | $ | 80,978.44     
|5/1/154/30/16  |  | $                 | 997,146.25 |                 | $ | 83,095.52     

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SCHEDULE
II TO FOURTH AMENDMENT
List of Furniture

|Description                     |  | QTY.
|------------------------------- |  | ----
|2 Drawer rolling Cabinets       |  | 38  
|Cube Vanity                     |  | 33  
|Overhead Cube Bins              |  | 23  
|Metal Side Chairs               |  | 41  
|Standard wood office sets       |  | 18  
|Lg. Executive office sets       |  | 4   
|Aeron Chairs                    |  | 26  
|Wood Side Chairs                |  | 25  
|Cafeteria Lounge Chairs         |  | 15  
|Cafeteria Lounge Sofa's         |  | 2   
|Green Lounge Chairs             |  | 3   
|42" round café tables           |  | 10  
|Steel Café Chairs               |  | 29  
|Metal Bar Stools                |  | 7   
|Stainless Steel refrig.         |  | 1   
|White refrig.                   |  | 1   
|Microwaves                      |  | 2   
|Red Conference Room Chairs      |  | 18  
|CR Table Approx. 20'x6'         |  | 1   
|Glass Top Credenza 6'x23"       |  | 1   
|Green Conference Room Chairs    |  | 12  
|CR Table Approx. 12'x54:        |  | 1   
|Credenza                        |  | 1   
|Beige Conference Room Chairs    |  | 12  
|CR Table 12'x54"                |  | 1   
|Gold Conference Room Chairs     |  | 8   
|Computer Training Tables        |  | 8   
|White Boards                    |  | 21  
|42" Round Pine Table            |  | 1   
|38"x23" pine credenzas          |  | 2   
|Beige 4 Shelf Bookcase          |  | 2   
|Grey Four Drawer laterals       |  | 3   
|Pine credenza/bookcase          |  | 4   
|Grey three draw pedestals       |  | 3   
|5'x2' Table                     |  | 1   
|Brown Rolling Credenza's 34x20  |  | 3   
|Brown & Grey Cred, 5'x18"       |  | 2   
|Brown & Black 6'x3' table       |  | 1   
|Wood Credenza 92"x 23"          |  | 1   
|Oval wood & Steel meeting table |  | 1   
|Large reception Cubes           |  | 2   
|Horseshoe training tables       |  | 2   
|Grey cube side tables           |  | 9   
|Red Reception Chairs            |  | 4   
|Glass Reception round table     |  | 1   
|8x8 Office Cubes                |  | 20  
|Bullpen type cubes              |  | 17  

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|7x9 grey & wood tables     |  | 2
|-------------------------- |  | -
|6'x24" pine credenza       |  | 1
|5'x20" pine credenza       |  | 1
|Grey 5 drawer file cabinet |  | 2
|Liebert cooling systems    |  | 3
|Liebert Backup power unit  |  | 1

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