Document:

<PAGE>   1
                                                                   EXHIBIT 10.50

                                THIRD AMENDMENT
            TO PROMISSORY NOTE IN THE PRINCIPAL AMOUNT OF $2,500,000
                                FIRST AMENDMENT
            TO PROMISSORY NOTE IN THE PRINCIPAL AMOUNT OF $2,500,000
                             AND FIFTH AMENDMENT TO
                              CREDIT AGREEMENT AND
                                     WAIVER

This Amendment and Waiver Agreement ("Amendment") is made as of March 23, 2000
by and between DENTAL/MEDICAL DIAGNOSTIC SYSTEMS, INC., ("Borrower") and
Imperial Bank, a California banking corporation, ("Bank") and amends certain
provisions of (i) that Promissory Note dated as of January 4, 1999 executed by
Borrower in favor of Bank in the principal amount of $2,500,000 ("Note 1"),
(ii) that Promissory Note dated as of January 4, 1999 executed by Borrower in
favor of Bank in the principal amount of $4,000,000 ("Note 2") and (iii) that
Credit Agreement dated January 4, 1999 executed by Borrower in favor of Bank
("Agreement") and contains certain waivers by Bank of the Borrower complying
with certain provisions of the Agreement as follows:

Whereas, Borrower violated certain financial covenants of the Agreement and has
requested Bank to waive such breaches.

Events of Default: The following Events of Defaults have occurred in the
Agreement:

A.   DEFAULTS AND WAIVERS. The Banks review of the Borrower's nine-month
interim financial statement dated December 31, 1999 disclosed the following
defaults have occurred in the Agreement as follows:

1.   Section 4.06 "QUICK RATIO. Maintain quick ratio of cash and accounts
receivable to current liabilities of at least 1.00:1.0."

Default: Actual Quick Ratio was .76:1.0.

2.   Section 4.08 "TANGIBLE NET WORTH. Maintain at all times a Tangible Net
Worth (defined as stockholder's equity loss any value for goodwill, trademarks,
patents, copyrights, leaseholds, organization expense and other similar
intangible items, and any amounts due from stockholders, officers and
affiliates) of not less than Ten Million Dollars ($10,000,000)."

Default: Actual Tangible Net Worth was $7,910,000.

3.   Section 4.09 "DEBT TO TANGIBLE NET WORTH. Maintain at all times a
consolidated ratio of total liabilities to Tangible Net Worth (defined as
stockholder's equity less any value for goodwill, trademarks, patents,
copyrights, leaseholds, organization expense and other similar intangible items,
and any amounts due from stockholders, officers and affiliates) of not greater
than 1.50:1.0."

                                       1

<PAGE>   2
Default:  Actual Debt to Tangible Net Worth was 1.75:1.0.

4.   Section 4.10 "FIXED CHARGE COVERAGE RATIO. Maintain quarterly, a Fixed
Charge Ratio of not less the 1.5:1.0."

Default:  Actual Fixed Charge Ratio was negative.

5.   Section 4.11 "PROFITABILITY, Maintain on a consolidated basis, profitable
operations (meaning a net profit after taxes) of at least $1.00 on a quarterly
basis beginning with the quarter ending 12/31/98."

Default:  Actual Profitability for the quarter ending 12/31/99 was a net loss
          of ($5,971,00).

The above waivers are specific as to contents and times, and other than the
waivers mentioned above this amendment is not a waiver of any other rights or
remedies that Bank may have pursuant to any agreement or law as a result of any
other violations past, present, or future of any agreement between the Borrower
and the Bank, and Bank reserves all rights, powers and remedies available to it.

1.   The above waivers are conditioned upon the following events, and the
failure of Borrower to comply with the following shall terminate this waiver and
result in the occurrence of an Event of Default under the Agreement, giving the
Banks all the rights and remedies thereunder:

(a)  Bank receiving a fee of $25,000 from the Borrower. Borrower hereby
authorizes bank to charge its account #10087228 with Bank for this fee.

(b)  Borrower agrees to supply Bank a list of all it's intellectual property
and provide Bank in form and substance satisfactory to Bank a perfected
security interest in all patents, trademarks and copyrights as deemed necessary
by Bank within 60 days of the date of this Amendment.

(c)  Borrower agrees to provide Bank with 75,000 warrants for Common Stock of
Borrower at the strike price of $2.50 and to include Conversion Rights (net
exercise right). Transferability to Imperial Bank Affiliates, minimum term of
seven (7) years, warrants to be registered, and Warrant issuance on Bank form
and in form and substance satisfactory to Bank.

(d)  Borrower agrees to amendments of the Credit Agreement and Promissory Notes
as described below.

                                       2
<PAGE>   3
A. AMENDMENTS TO THE PROMISSORY NOTE IN THE ORIGINAL AMOUNT OF $2,500,000.

1.   The Note 1 is amended by deleting, in its entirety, the section entitled
PAYMENT and replacing it with the following:

"PAYMENT. Subject to any payment changes resulting from changes in the Index,
Borrower will pay this loan in accordance with the following payment schedule:

"Fourteen principal payments of $100,000.00 each on March 31, 2000, April 30,
2000, May 31, 2000, June 30, 2000, July 31, 2000, August 31, 2000, September 30,
2000, October 31, 2000, November 30, 2000, December 31, 2000, January 31, 2001,
February 28, 2001, March 31, 2001 and April 30, 2001 with interest calculated on
the unpaid principal balances at an interest rate of 1.0 percentage point over
the Index, described above and one final principal payment of $900,000 on May
31, 2001. This estimated final payment is based on the assumption that all
payments will be made exactly as scheduled and that the Index does not change;
the actual final payment will be for all principal and accrued interest not yet
paid, together with any other unpaid amounts under this Note."

2. Note 1 is further amended by deleting "The interest rate to be applied to the
unpaid principal balance in this Note will be at a rate of 0.5 percentage points
over the Index, resulting in an initial rate of 9.0%." and replaced with the
following:

"The interest rate to be applied to the unpaid principal balance in this Note
will be at a rate of 1.0 percentage point over the Index, resulting in an
initial rate of 10.0%."

B. AMENDMENT TO THE PROMISSORY NOTE IN THE ORIGINAL AMOUNT OF $4,000,000 ("NOTE
2").

1. Note 2 is amended by amending the section entitled PAYMENT deleting the
sentence "Borrower will pay this loan in one payment of all outstanding
principal plus all accrued unpaid interest on May 31, 2000" and replacing it
with the following:

"Borrower will pay this loan in one payment of all outstanding principal plus
all accrued unpaid interest on May 31, 2001."

2. Note 2 is further amended by amending the section entitled VARIABLE INTEREST
RATE. The sentence "The interest rate to be applied to the unpaid principal
balance of this Note will be at a rate of 0.5000 percentage point over the
Index, resulting in an initial rate of 8.250%" shall be deleted and replaced
with the following:

"The interest rate to be applied to the unpaid principal balance of this Note
will be at a rate of 1.00 percentage point over the Index, resulting in an
initial rate of 10.0%."

                                       3

<PAGE>   4
C.    AMENDMENTS TO THE CREDIT AGREEMENT

1.    Section 1.03 REVOLVING LINE OF CREDIT is amended by deleting "until May
      31, 2000 (the "Revolving Line of Credit Maturity Date")" and replacing
      with "until May 31, 2001 (the "Revolving Line of Credit Maturity Date")".

2.    Section 4.07 QUICK RATIO is amended by adding the following:

      "Current liabilities shall include all amounts outstanding to Bank which
      are due under the $4,000,000 revolving line of credit and the Term A Loan
      as stated on the Credit Agreement dated January 4, 1999 regardless of
      maturity date and GAAP presentation."

3.    Section 4.09 TANGIBLE NET WORTH is amended by adding the following
      sentence:

      "Tangible Net Worth shall be increased by 75% of the amount of equity
      raised beginning 4/1/00."

4.    Section 4.11 PROFITABILITY shall be amended by deleting the phrase "with
      the quarter ending 12/31/98" and shall be replaced by the following:

      "with the quarter ending June 30, 2000."

D.    Except as provided above, the Agreement and Note 1 and Note 2 remain
      unchanged. Any capitalized terms used herein and not defined herein shall
      have the meanings defined in the Agreement.

DENTAL/MEDICAL DIAGNOSTIC SYSTEMS, INC.

By:   /s/ STEPHEN ROSS
     -----------------------
     Stephen Ross

Its: Chief Financial Officer

IMPERIAL BANK

By:   /s/ TRICIA HUNTER
     ------------------------------------
     Tricia Hunter, Senior Vice President

                                       4<PAGE>   1

                            SANGAMO BIOSCIENCES, INC.

                      INCORPORATED IN THE STATE OF DELAWARE

COMMON SHARES                                                     COMMON SHARES

THIS CERTIFICATE IS TRANSFERABLE                               CUSIP 80677 10 6
IN BOSTON, MA AND NEW YORK, NY               SEE REVERSE FOR CERTAIN DEFINITIONS

This certifies that

is the recordholder of

            FULLY PAID AND NONASSESSABLE SHARES OF THE COMMON STOCK,
                          PAR VALUE $0.01 PER SHARE of

Sangamo BioSciences, Inc. transferable on the books of the Corporation by the
holder hereof in person or by duly authorized Attorney upon surrender of this
Certificate properly endorsed. This Certificate is not valid until
countersigned and registered by the Transfer Agent and Registrar.

/s/ Shawn Johnson                                     /s/ Edward O. Lanphier
      Secretary                        President and Chief Executive Officer

Countersigned and Registered
        EQUISERVE TRUST COMPANY, N.A.

By: [illegible]
    ------------------------
    Authorized Signature

<PAGE>   2

                           SANGAMO BIOSCIENCES, INC.

     Upon request the Corporation will furnish any holder of shares of Common
Stock of the Corporation, without charge, with a full statement of the powers,
designations, preferences, and relative, participating, optional or other
special rights of any class or series of capital stock of this Corporation, and
the qualifications, limitations or restrictions of such preferences and/or
rights.

     The following abbreviations, when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

TEN COM -- as tenants in common
TEN ENT -- as tenants by the entireties
JT TEN  -- as joint tenants with right of
           survivorship and not as tenants in common

UNIF GIFT MIN ACT --                            Custodian
                     --------------------------           ----------------------
                             (cust)                              (minor)
                     under Uniform Gift to Minor
                     Act
                         -----------------------
                                (state)

   Additional abbreviations may also be used although not in the above item.

For Value received, ___________________________ hereby sell, assign and transfer
unto
PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE
______________________________________

________________________________________________________________________________
(PLEASE PRINT OR TYPEWRITE THE NAME AND ADDRESS, INCLUDING ZIP CODE OF ASSIGNEE)
________________________________________________________________________________

________________________________________________________________________________

_________________________________________________________________________ Shares

of Common Stock represented by the within Certificate, and to hereby irrevocably
constitute and appoint
_______________________________________________________________________ Attorney
to transfer the said stock on the books of the within named Corporation with
full power of substitution in the premises.

Dated ____________________

In presence of

X _______________________________   X ________________________________
                                          THE SIGNATURE TO THE ASSIGMENT
                                          MUST CORRESPOND WITH THE NAME AS
                                  NOTICE: WRITTEN UPON THE FACE OF THE
                                          CERTIFICATE IN EVERY PARTICULAR,
                                          WITHOUT ALTERATION OR ENLARGEMENT OR
                                          ANY CHANGE WHATEVER.

Signature(s) Guaranteed

By ______________________________
THE SIGNATURE(S) MUST BE GUARANTEED BY AN ELIGIBLE
GUARANTOR INSTITUTION (BANK, STOCKBROKER, SAVINGS
AND LOAN ASSOCIATION AND CREDIT UNION WITH MEMBERSHIP
IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM),
PURSUANT TO SEC RULE 17 AND 18.

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00006-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00006-of-00352.parquet"}]]