Document:

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EXHIBIT 10.56

January 21, 2000

Timothy J. Rink, M.D., Sc.D.
Chairman
Aurora Biosciences Corp.
Les Ligures Apt. 612
2, Rue Honore Labande
MC 98000  Monaco

Dear Tim:

This letter agreement ("Agreement") sets forth certain terms relating to your
service as Chairman of the Company's Board of Directors (the "Board"). This
Agreement will be deemed effective January 1, 2000.

-    CHAIRMANSHIP OF BOARD OF DIRECTORS. Beginning January 1, 2000 and as long
     as you serve as Chairman of the Board, you will receive fees for meetings
     comparable to the fees received by other non-executive directors, currently
     $2,500.00 per Board meeting attended by you in person, $500.00 per
     telephone or committee meeting in which you participate.

     You will also receive a quarterly Chairman's fee of $2,500.00, payable in
     advance on the first day of each quarter for as long as you remain Chairman
     of the Board.

-    You may provide consulting services from time to time to the Company, as
     follows.  You will receive a fee of $2,500.00 per day of service on behalf
     of the Company, additional to the days of Board meetings.

          -    Areas of service may include: recruitment; customer relations;
               business development; strategic alliances; investor relations;
               organization development; scientific and technical review and any
               other areas in which the Board requests your services.

          -    You will not be requested to make more than three visits to the
               USA in any quarter.

          -    Expenses: The Company will pay reasonable and documented travel
               and incidental business expenses including: business-class air
               travel for intercontinental flights for business performed in the
               USA at the Company's request, car rental and hotel-suite
               accommodations for work periods in San Diego.

          -    Secretarial assistance and other facilities: You will receive
               reasonable secretarial assistance and office facilities at the
               Company's San Diego headquarters.

          -    COBRA: The Company will reimburse you for the costs of your COBRA
               coverage or the premiums for medical coverage similar to your
               Company sponsored medical coverage (excluding family members)
               during your service as a consultant.

          -    You agree that, through June 30th, 2000, so far as reasonably
               practicable the Company's business will take precedence over any
               other professional work.

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          -    Other professional activities: You agree that during the term of
               your consultancy any third party work shall not conflict with the
               Company's business interests. You further agree to notify the CEO
               of the Company of the identity and nature of work involved in any
               third party affiliations during the term of your consultancy.

          -    Stock Options: All of the stock options to purchase Company
               Common Stock which have been granted to you and dated February
               18, 1997 and July 23, 1998 will continue to vest in accordance
               with their terms and the Company's 1996 Stock Plan as long as you
               continue to serve as a member of the Board of Directors of, or as
               a consultant to, the Company.

          -    Employee Proprietary Information and Inventions Agreement: The
               conditions of the Employee Proprietary Information & Inventions
               Agreement you executed on February 20, 1996, and attached hereto
               as Attachment A, will remain in effect as provided therein.

          -    This Agreement is subject to satisfactory proof of your right to
               work in the USA.

          -    Your service as a consultant may be terminated by you or the
               Company at any time upon written notice to the other party.

- You will receive a fee of $5,000.00 whenever the company requests you to
  travel to the USA on behalf of the company.

Aurora may be required to withhold applicable U.S. federal and state taxes from
payments made to you for services provided in the U.S. At the end of each year,
the Company will send you the appropriate U.S. federal and state forms reporting
the amount of payments for services provided in the U.S. You are responsible for
all tax filings and tax payments.

This Agreement is made in San Diego, California. This Agreement shall be
construed and interpreted in accordance with the laws of the State of
California. Once signed by you, this Agreement will constitute the complete,
final and exclusive agreement between you and the Company relating to the terms
and conditions of your employment, and supersedes all prior and contemporaneous
oral and written employment agreements or arrangements between the company and
you.

Yours sincerely,

/s/ JAMES BLAIR                                    /s/ HUGH RIENHOFF, JR.
----------------------------                       -----------------------------
James Blair, Ph.D.                                 Hugh Rienhoff, Jr., M.D.
Compensation Committee of the Board of Directors

cc:      S. Collinson, President
         P. Fritz, Senior Director, Human Resources
         J. Pashkowsky, Senior Director, Finance and Treasurer

I accept the terms and conditions outlined in this Agreement.

/s/ TIMOTHY J. RINK
-----------------------------
Timothy J. Rink, M.D., Sc.D.<PAGE>

                                                                  EXHIBIT 10.6

                                 AMENDMENT NO. 4
             (dated September 28, 1999 for reference purposes only)

                         To Lease Dated April 15, 1994

               Between PRESBYTERIAN HEALTH FOUNDATION as Landlord,
       and UROCOR, INC. formerly known as CytoDiagnostics, Inc., as Tenant

     WHEREAS, Landlord and Tenant are the parties to the above described Lease
for the Building located at 840 N. Research Parkway; and,

Landlord and Tenant mutually desire to amend the above-described lease,
(amended via Amendment No. 1 dated April 14, 1997, Amendment No. 2 dated
April 14, 1998 and Amendment No. 3 dated February 1, 1999) as follows:

The provisions of this Amendment No. 4 shall supersede and control any
inconsistent provisions contained in the Lease, regardless of whether such
inconsistent provisions are contained in the printed portion of the Lease or
any rider, addendum, amendment or exhibit annexed thereto.

Amended Leased Premises:
The Leased Premises shall increase as indicated by the following schedule:

<TABLE>
<CAPTION>
                                                   Square Feet                  Room             Lease Commencement
                                                   -----------                 ------           --------------------
                                                   <S>                         <C>              <C>
                                                       146                       539                      10/01/99
                                                       146                       540                      10/01/99
                                                       146                       541                      10/01/99
                                                       2615                      455                      10/01/99
                                                       176                       next to 270              04/01/00
                                                       359                       303                      04/01/00
                                                       354                       359                      04/01/00
                                                       473                       358                      04/01/00
                                                       111                       411                      04/01/00
                                                       278                       401                      04/01/00
                                                       620                       East side of 455         04/01/00
                                                       140                       519                      04/01/00
                                                       700                       171                      12/01/00

</TABLE>
                    See the attached floor plans for room locations.

Commencing October 1, 1999, the Leased Premises increases by 3,053 square
feet = 101,965 gross sq. ft.; then commencing April 1, 2000, the Leased
Premises shall further increase by 2,511 square feet = 104,476 gross sq. ft.;
then commencing December 1, 2000, the Leased Premises shall further increase
by 700 square feet = 105,176 gross sq. ft.

Amended Tenant Improvement
Allowance:                        Limited to $25.65 per rentable square foot for
                                  the 3,053 rentable square feet (rooms:  455
                                  [2,615  square  feet], 539, 540 & 541) and
                                  $24.60 for the 2,511 rentable square feet
                                  (rooms:  next to 270, 303, 359, 358, 411, 401,
                                  east side of 455 [620 square feet] & 519) and
                                  $23.55 for 700 rentable square feet (room:
                                  171).  Architectural fees are paid from the
                                  Tenant Improvement Allowance.  Any excess cost
                                  of Tenant Improvements shall be the
                                  responsibility of the Tenant.

Both parties agree to the terms of the Lease Agreement as amended by Amendment
No. 1, Amendment No. 2, Amendment No. 3 and this Amendment No. 4, and the Lease
Agreement shall continue in full force and effect.

Landlord:         Presbyterian Health Foundation

By:                /s/ Dennis McGrath                    Date:  11/24/99
                   -----------------------------                --------------
                  Dennis McGrath, Vice President

Tenant:  UROCOR, INC.

By:                /s/ Michael W. George                 Date:  10/28/99
                   -----------------------------                ---------------
                   Michael W. George
                   President/Chief Operating Officer

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