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                                                                     EXHIBIT 4.1

                                                                       EXHIBIT B
                                                                   TO SECURITIES
                                                                        PURCHASE
                                                                       AGREEMENT

        VOID AFTER 5:00 P.M., NEW YORK CITY
        TIME, ON NOVEMBER 19, 2007
        (UNLESS EXTENDED PURSUANT TO SECTION 2 HEREOF)

        THIS WARRANT AND THE SHARES ISSUABLE UPON EXERCISE OF THIS WARRANT HAVE
        NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE
        "SECURITIES ACT"), OR THE SECURITIES LAWS OF ANY STATE OF THE UNITED
        STATES OR ANY OTHER JURISDICTION. THE SECURITIES REPRESENTED HEREBY MAY
        NOT BE OFFERED, SOLD OR TRANSFERRED IN THE ABSENCE OF AN EFFECTIVE
        REGISTRATION STATEMENT FOR THE SECURITIES UNDER APPLICABLE SECURITIES
        LAWS UNLESS OFFERED, SOLD OR TRANSFERRED PURSUANT TO AN AVAILABLE
        EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THOSE LAWS.

                                         Right to Purchase ________ Shares of
                                         Common Stock, par value $.001 per share

Date: November 19, 2002

                                 VIROLOGIC, INC.
                             STOCK PURCHASE WARRANT

        THIS CERTIFIES THAT, for value received, _______________________, or its
registered assigns, is entitled to purchase from ViroLogic, Inc., a corporation
organized under the laws of the State of Delaware (the "COMPANY"), at any time
or from time to time during the period specified in Section 2 hereof,
_____________________________________ fully paid and nonassessable shares of the
Company's common stock, par value $.001 per share (the "COMMON STOCK"), at an
exercise price per share (the "EXERCISE PRICE") equal to $1.11. The number of
shares of Common Stock purchasable hereunder (the "WARRANT SHARES") and the
Exercise Price are subject to adjustment as provided in Section 4 hereof. The
term "WARRANTS" means this Warrant and the other warrants of the Company issued
pursuant to that certain Securities Purchase Agreement, dated as of November 14,
2002, by and among the Company and the other signatories thereto (the
"SECURITIES PURCHASE AGREEMENT").

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        This Warrant is subject to the following terms, provisions and
conditions:

        1. Manner of Exercise; Issuance of Certificates; Payment for Shares.
Subject to the provisions hereof, including, without limitation, the limitations
contained in Section 7 hereof, this Warrant may be exercised by the holder
hereof, in whole or in part, by the surrender of this Warrant, together with a
completed exercise agreement in the form attached hereto (the "EXERCISE
AGREEMENT"), to the Company during normal business hours on any business day at
the Company's principal executive offices (or such other office or agency of the
Company as it may designate by notice to the holder hereof), and (i) payment to
the Company in cash, by certified or official bank check or by wire transfer for
the account of the Company, of the Exercise Price for the Warrant Shares
specified in the Exercise Agreement or (ii) if the holder is effectuating a
Cashless Exercise (as defined in Section 11(c) hereof) pursuant to Section 11(c)
hereof, delivery to the Company of a written notice of an election to effect a
Cashless Exercise for the Warrant Shares specified in the Exercise Agreement.
The Warrant Shares so purchased shall be deemed to be issued to the holder
hereof or such holder's designee, as the record owner of such shares, as of the
close of business on the date on which this Warrant shall have been surrendered,
the completed Exercise Agreement shall have been delivered, and payment shall
have been made for such shares as set forth above or, if such date is not a
business date, on the next succeeding business date. The Warrant Shares so
purchased, representing the aggregate number of shares specified in the Exercise
Agreement, shall (by the Company or through its transfer agent) be delivered
(i.e., deposited with a nationally-recognized overnight courier service postage
prepaid) to the holder hereof within a reasonable time, not exceeding two
business days, after this Warrant shall have been so exercised (the "DELIVERY
PERIOD"). If the Company's transfer agent is participating in the Depository
Trust Company ("DTC") Fast Automated Securities Transfer program, and so long as
the certificates therefor do not bear a legend (pursuant to the terms of the
Securities Purchase Agreement) and the holder is not obligated to return such
certificate for the placement of a legend thereon (pursuant to the terms of the
Securities Purchase Agreement), the Company shall cause its transfer agent to
electronically transmit the Warrant Shares so purchased to the holder by
crediting the account of the holder or its nominee with DTC through its Deposit
Withdrawal Agent Commission system ("DTC TRANSFER"). If the aforementioned
conditions to a DTC Transfer are not satisfied, the Company shall deliver as
provided herein to the holder physical certificates representing the Warrant
Shares so purchased. Further, the holder may instruct the Company to deliver to
the holder physical certificates representing the Warrant Shares so purchased in
lieu of delivering such shares by way of DTC Transfer. Any certificates so
delivered shall be in such denominations as may be reasonably requested by the
holder hereof, shall be registered in the name of such holder or such other name
as shall be designated by such holder and, following the date on which the
Warrant Shares have been registered under the Securities Act pursuant to that
certain Registration Rights Agreement, dated as of November 19, 2002, by and
between the Company and the other signatories thereto (the "REGISTRATION RIGHTS
AGREEMENT") or otherwise may be sold by the holder pursuant to Rule 144
promulgated under the Securities Act (or a successor rule), shall not bear any
restrictive legend. If this Warrant shall have been exercised only in part, then
the Company shall, at its expense, at the time of delivery of such certificates,
deliver to the holder a new Warrant representing the number of shares with
respect to which this Warrant shall not then have been exercised.

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        If, at any time, a holder of this Warrant submits this Warrant, an
Exercise Agreement and payment to the Company of the Exercise Price for each of
the Warrant Shares specified in the Exercise Agreement (including pursuant to a
Cashless Exercise), and the Company fails for any reason to deliver, on or prior
to the fourth business day following the expiration of the Delivery Period for
such exercise, the number of shares of Common Stock to which the holder is
entitled upon such exercise (an "EXERCISE DEFAULT"), then the Company shall pay
to the holder payments ("EXERCISE DEFAULT PAYMENTS") for an Exercise Default in
the amount of (a) (N/365), multiplied by (b) the amount by which the Market
Price (as defined in Section 4(l) hereof) on the date the Exercise Agreement
giving rise to the Exercise Default is transmitted in accordance with this
Section 1 (the "EXERCISE DEFAULT DATE") exceeds the Exercise Price in respect of
such Warrant Shares, multiplied by (c) the number of shares of Common Stock the
Company failed to so deliver in such Exercise Default, multiplied by (d) .24,
where N = the number of days from the Exercise Default Date to the date that the
Company effects the full exercise of this Warrant which gave rise to the
Exercise Default. The accrued Exercise Default Payment for each calendar month
shall be paid in cash and shall be made to holder by the fifth day of the month
following the month in which it has accrued. Nothing herein shall limit the
holder's right to pursue actual damages for the Company's failure to maintain a
sufficient number of authorized shares of Common Stock as required pursuant to
the terms of Section 3(b) hereof or to otherwise issue shares of Common Stock
upon exercise of this Warrant in accordance with the terms hereof, and the
holder shall have the right to pursue all remedies available at law or in equity
(including a decree of specific performance and/or injunctive relief).

        2. Period of Exercise. This Warrant shall be exercisable at any time or
from time to time during the period beginning on the date six months and one day
from the date of initial issuance of this Warrant (the "ISSUE DATE") and ending
at 5:00 p.m., New York City time, on the fifth anniversary of the Issue Date
(the "EXERCISE PERIOD"). The Exercise Period shall automatically be extended by
one (1) day for each day on which the Company does not have a number of shares
of Common Stock reserved for issuance upon exercise hereof at least equal to the
number of shares of Common Stock issuable upon exercise hereof.

        3. Certain Agreements of the Company. The Company hereby covenants and
agrees as follows:

               (a) Shares to be Fully Paid. All Warrant Shares will, upon
issuance in accordance with the terms of this Warrant, be validly issued, fully
paid, and nonassessable and free from all taxes, liens, claims and encumbrances.

               (b) Reservation of Shares. During the Exercise Period, the
Company shall at all times have authorized, and reserved for the purpose of
issuance upon exercise of this Warrant, a sufficient number of shares of Common
Stock to provide for the exercise in full of this Warrant (without giving effect
to the limitations on exercise set forth in Section 7(g) hereof).

               (c) Listing. The Company shall promptly secure the listing of the
shares of Common Stock issuable upon exercise of this Warrant upon each national
securities exchange or automated quotation system, if any, upon which shares of
Common Stock are then listed or become listed (subject to official notice of
issuance upon exercise of this Warrant) and shall maintain, so

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long as any other shares of Common Stock shall be so listed, such listing of all
shares of Common Stock from time to time issuable upon the exercise of this
Warrant; and the Company shall so list on each national securities exchange or
automated quotation system, as the case may be, and shall maintain such listing
of, any other shares of capital stock of the Company issuable upon the exercise
of this Warrant if and so long as any shares of the same class shall be listed
on such national securities exchange or automated quotation system.

               (d) Certain Actions Prohibited. The Company will not, by
amendment of its charter or through any reorganization, transfer of assets,
consolidation, merger, dissolution, issue or sale of securities, or any other
voluntary action, avoid or seek to avoid the observance or performance of any of
the terms to be observed or performed by it hereunder, but will at all times in
good faith assist in the carrying out of all the provisions of this Warrant and
in the taking of all such action as may reasonably be requested by the holder of
this Warrant in order to protect the economic benefit inuring to the holder
hereof and the exercise privilege of the holder of this Warrant against dilution
or other impairment, consistent with the tenor and purpose of this Warrant.
Without limiting the generality of the foregoing, the Company (i) will not
increase the par value of any shares of Common Stock receivable upon the
exercise of this Warrant above the Exercise Price then in effect, and (ii) will
take all such actions as may be necessary or appropriate in order that the
Company may validly and legally issue fully paid and nonassessable shares of
Common Stock upon the exercise of this Warrant.

               (e) Successors and Assigns. This Warrant will be binding upon any
entity succeeding to the Company by merger, consolidation, or acquisition of all
or substantially all of the Company's assets.

               (f) Blue Sky Laws. The Company shall, on or before the date of
issuance of any Warrant Shares, take such actions as the Company shall
reasonably determine are necessary to qualify the Warrant Shares for, or obtain
exemption for the Warrant Shares for, sale to the holder of this Warrant upon
the exercise hereof under applicable securities or "blue sky" laws of the states
of the United States, and shall provide evidence of any such action so taken to
the holder of this Warrant prior to such date; provided, however, that the
Company shall not be required in connection therewith or as a condition thereto
to (a) qualify to do business in any jurisdiction where it would not otherwise
be required to qualify but for this Section 3(f), (b) subject itself to general
taxation in any such jurisdiction or (c) file a general consent to service of
process in any such jurisdiction.

        4. Antidilution Provisions. During the Exercise Period, the Exercise
Price and the number of Warrant Shares issuable hereunder shall be subject to
adjustment from time to time as provided in this Section 4.

               In the event that any adjustment of the Exercise Price as
required herein results in a fraction of a cent, such Exercise Price shall be
rounded up or down to the nearest cent.

               (a) [Intentionally Omitted]

               (b) [Intentionally Omitted]

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               (c) Subdivision or Combination of Common Stock. If the Company,
at any time during the Exercise Period, subdivides (by any stock split, stock
dividend, recapitalization, reorganization, reclassification or otherwise) its
shares of Common Stock into a greater number of shares, then, after the date of
record for effecting such subdivision, the Exercise Price in effect immediately
prior to such subdivision will be proportionately reduced. If the Company, at
any time during the Exercise Period, combines (by reverse stock split,
recapitalization, reorganization, reclassification or otherwise) its shares of
Common Stock into a smaller number of shares, then, after the date of record for
effecting such combination, the Exercise Price in effect immediately prior to
such combination will be proportionately increased.

               (d) Adjustment in Number of Shares. Upon each adjustment of the
Exercise Price pursuant to the provisions of this Section 4, the number of
shares of Common Stock issuable upon exercise of this Warrant at each such
Exercise Price shall be adjusted by multiplying a number equal to the Exercise
Price in effect immediately prior to such adjustment by the number of shares of
Common Stock issuable upon exercise of this Warrant at such Exercise Price
immediately prior to such adjustment and dividing the product so obtained by the
adjusted Exercise Price.

               (e) Consolidation, Merger or Sale. In case of (i) any
consolidation of the Company with, or merger of the Company into, any other
entity, (ii) any sale or conveyance of all or substantially all of the assets of
the Company other than in connection with a plan of complete liquidation of the
Company or (iii) any share exchange pursuant to which all of the outstanding
shares of Common Stock are converted into other securities or property (each of
(i) - (iii) above being a "CORPORATE CHANGE") at any time during the Exercise
Period, then as a condition of such Corporate Change, adequate provision will be
made whereby the holder hereof will have the right to acquire and receive upon
exercise of this Warrant in lieu of the shares of Common Stock immediately
theretofore acquirable upon the exercise of this Warrant, such shares of stock,
securities, cash or assets as may be issued or payable with respect to or in
exchange for the number of shares of Common Stock immediately theretofore
acquirable and receivable upon exercise of this Warrant had such Corporate
Change not taken place. In any such case, the Company will make appropriate
provision to insure that the provisions of this Section 4 hereof will thereafter
be applicable as nearly as may be in relation to any shares of stock or
securities thereafter deliverable upon the exercise of this Warrant. The Company
will not effect any Corporate Change unless prior to the consummation thereof,
the successor corporation (if other than the Company) assumes by written
instrument the obligations under this Warrant and the obligations to deliver to
the holder hereof such shares of stock, securities or assets as, in accordance
with the foregoing provisions, the holder may be entitled to acquire.
Notwithstanding the foregoing, in the event of any Corporate Change at any time
during the Exercise Period, the holder hereof shall, at its option, have the
right to receive, in connection with such transaction, cash consideration equal
to the fair market value of this Warrant as determined in accordance with
customary valuation methodology used in the investment banking industry.

               (f) Distribution of Assets. In case the Company shall declare or
make any distribution of its assets (or rights to acquire its assets) to holders
of Common Stock as a partial liquidating dividend, stock repurchase by way of
return of capital or otherwise (including any dividend or distribution to the
Company's stockholders of cash or shares (or rights to acquire shares) of
capital stock of a subsidiary) (a "DISTRIBUTION"), at any time during the
Exercise Period, then the

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holder hereof shall be entitled upon exercise of this Warrant for the purchase
of any or all of the shares of Common Stock subject hereto, to receive the
amount of such assets (or rights) which would have been payable to the holder
had such holder been the holder of such shares of Common Stock on the record
date for the determination of stockholders entitled to such Distribution. If the
Company distributes rights, warrants, options or any other form of convertible
securities and the right to exercise or convert such securities would expire in
accordance with their terms prior to the expiration of the Exercise Period, then
the terms of such securities shall provide that such exercise or convertibility
right shall remain in effect until 30 days after the date the holder hereof
receives such securities pursuant to the exercise hereof.

               (g) Notice of Adjustment. Upon the occurrence of any event which
requires any adjustment of the Exercise Price, then, and in each such case, the
Company shall give notice thereof to the holder hereof, which notice shall state
the Exercise Price resulting from such adjustment and the increase or decrease
in the number of Warrant Shares purchasable at such price upon exercise, setting
forth in reasonable detail the method of calculation and the facts upon which
such calculation is based. Such calculation shall be certified by the chief
financial officer of the Company.

               (h) Minimum Adjustment of Exercise Price. No adjustment of the
Exercise Price shall be made in an amount of less than $.01, but any such lesser
adjustment shall be carried forward and shall be made at the time and together
with the next subsequent adjustment which, together with any adjustments so
carried forward, shall amount to not less than $.01.

               (i) No Fractional Shares. No fractional shares of Common Stock
are to be issued upon the exercise of this Warrant, but the Company shall pay a
cash adjustment in respect of any fractional share which would otherwise be
issuable in an amount equal to the same fraction of the Market Price of a share
of Common Stock on the date of such exercise.

               (j) Other Notices. In case at any time:

                      (i) the Company shall declare any dividend upon the Common
Stock payable in shares of stock of any class or make any other distribution
(other than dividends or distributions payable in cash out of retained earnings
consistent with the Company's past practices with respect to declaring dividends
and making distributions) to the holders of the Common Stock;

                      (ii) the Company shall offer for subscription pro rata to
the holders of the Common Stock any additional shares of stock of any class or
other rights;

                      (iii) there shall be any capital reorganization of the
Company, or reclassification of the Common Stock, or consolidation or merger of
the Company with or into, or sale of all or substantially all of its assets to,
another corporation or entity; or

                      (iv) there shall be a voluntary or involuntary
dissolution, liquidation or winding-up of the Company;

then, in each such case, the Company shall give to the holder of this Warrant
(a) notice of the date or estimated date on which the books of the Company shall
close or a record shall be taken for determining the holders of Common Stock
entitled to receive any such dividend, distribution, or

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subscription rights or for determining the holders of Common Stock entitled to
vote in respect of any such reorganization, reclassification, consolidation,
merger, sale, dissolution, liquidation or winding-up and (b) in the case of any
such reorganization, reclassification, consolidation, merger, sale, dissolution,
liquidation or winding-up, notice of the date (or, if not then known, a
reasonable estimate thereof by the Company) when the same shall take place. Such
notice shall also specify the date on which the holders of Common Stock shall be
entitled to receive such dividend, distribution, or subscription rights or to
exchange their Common Stock for stock or other securities or property
deliverable upon such reorganization, reclassification, consolidation, merger,
sale, dissolution, liquidation, or winding-up, as the case may be. Such notice
shall be given at least fifteen (15) days prior to the record date or the date
on which the Company's books are closed in respect thereto. Failure to give any
such notice or any defect therein shall not affect the validity of the
proceedings referred to in clauses (i), (ii), (iii) and (iv) above.
Notwithstanding the foregoing, the Company shall publicly disclose the substance
of any notice delivered hereunder prior to delivery of such notice to the holder
hereof.

               (k) Certain Events. If, at any time during the Exercise Period,
any event occurs of the type contemplated by the adjustment provisions of this
Section 4 but not expressly provided for by such provisions, the Company will
give notice of such event as provided in Section 4(g) hereof, and an appropriate
adjustment in the Exercise Price and the number of shares of Common Stock
acquirable upon exercise of this Warrant at each such Exercise Price shall be
made so that the rights of the holder shall be neither enhanced nor diminished
by such event.

               (l) Certain Definitions.

                      (i) "MARKET PRICE," as of any date, (i) means the average
of the closing sales prices for the shares of Common Stock on the Nasdaq
National Market or other trading market where such security is listed or traded
as reported by Bloomberg Financial Markets (or a comparable reporting service of
national reputation selected by the Company and reasonably acceptable to the
holders if Bloomberg Financial Markets is not then reporting sales prices of
such security)(collectively, "BLOOMBERG") for the ten (10) consecutive trading
days immediately preceding such date, or (ii) if the Nasdaq National Market is
not the principal trading market for the shares of Common Stock, the average of
the reported sales prices reported by Bloomberg on the principal trading market
for the Common Stock during the same period, or, if there is no sales price for
such period, the last sales price reported by Bloomberg for such period, or
(iii) if the foregoing do not apply, the last sales price of such security in
the over-the-counter market on the pink sheets or bulletin board for such
security as reported by Bloomberg, or if no sales price is so reported for such
security, the last bid price of such security as reported by Bloomberg, or (iv)
if market value cannot be calculated as of such date on any of the foregoing
bases, the Market Price shall be the average fair market value as reasonably
determined by an investment banking firm selected by the Company and reasonably
acceptable to the holder, with the costs of the appraisal to be borne by the
Company. The manner of determining the Market Price of the Common Stock set
forth in the foregoing definition shall apply with respect to any other security
in respect of which a determination as to market value must be made hereunder.

                      (ii) "COMMON STOCK," for purposes of this Section 4,
includes the Common Stock and any additional class of stock of the Company
having no preference as to

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dividends or distributions on liquidation, provided that the shares purchasable
pursuant to this Warrant shall include only Common Stock in respect of which
this Warrant is exercisable, or shares resulting from any subdivision or
combination of such Common Stock, or in the case of any reorganization,
reclassification, consolidation, merger, or sale of the character referred to in
Section 4(e) hereof, the stock or other securities or property provided for in
such Section.

        5. Issue Tax. The issuance of certificates for Warrant Shares upon the
exercise of this Warrant shall be made without charge to the holder of this
Warrant or such shares for any issuance tax or other costs in respect thereof,
provided that the Company shall not be required to pay any tax which may be
payable in respect of any transfer involved in the issuance and delivery of any
certificate in a name other than the holder of this Warrant.

        6. No Rights or Liabilities as a Stockholder. This Warrant shall not
entitle the holder hereof to any voting rights or other rights as a stockholder
of the Company. No provision of this Warrant, in the absence of affirmative
action by the holder hereof to purchase Warrant Shares, and no mere enumeration
herein of the rights or privileges of the holder hereof, shall give rise to any
liability of such holder for the Exercise Price or as a stockholder of the
Company, whether such liability is asserted by the Company or by creditors of
the Company.

        7. Transfer, Exchange and Replacement of Warrant.

               (a) Restriction on Transfer. This Warrant and the rights granted
to the holder hereof are transferable, in whole or in part, upon surrender of
this Warrant, together with a properly executed assignment in the form attached
hereto, at the office or agency of the Company referred to in Section 7(e)
below, provided, however, that any transfer or assignment shall be subject to
the conditions set forth in Sections 7(f) and (g) hereof and to the provisions
of Sections 2(f) and 2(g) of the Securities Purchase Agreement. Until due
presentment for registration of transfer on the books of the Company, the
Company may treat the registered holder hereof as the owner and holder hereof
for all purposes, and the Company shall not be affected by any notice to the
contrary. Notwithstanding anything to the contrary contained herein, the
registration rights described in Section 8 hereof are assignable only in
accordance with the provisions of the Registration Rights Agreement.

               (b) Warrant Exchangeable for Different Denominations. This
Warrant is exchangeable, upon the surrender hereof by the holder hereof at the
office or agency of the Company referred to in Section 7(e) below, for new
warrants of like tenor of different denominations representing in the aggregate
the right to purchase the number of shares of Common Stock which may be
purchased hereunder, each of such new warrants to represent the right to
purchase such number of shares (at the Exercise Price therefor) as shall be
designated by the holder hereof at the time of such surrender, and all such
warrants thereafter constituting the Warrant referenced herein.

               (c) Replacement of Warrant. Upon receipt of evidence reasonably
satisfactory to the Company of the loss, theft, destruction, or mutilation of
this Warrant and, in the case of any such loss, theft, or destruction, upon
delivery of an indemnity agreement reasonably satisfactory in form and amount to
the Company, or, in the case of any such mutilation, upon surrender and
cancellation

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of this Warrant, the Company, at its expense, will execute and deliver, in lieu
thereof, a new Warrant of like tenor.

               (d) Cancellation; Payment of Expenses. Upon the surrender of this
Warrant in connection with any transfer, exchange, or replacement as provided in
this Section 7, this Warrant shall be promptly canceled by the Company. The
Company shall pay all taxes (other than securities transfer taxes) and all other
expenses (other than legal expenses, if any, incurred by the Holder or
transferees) and charges payable in connection with the preparation, execution,
and delivery of any Warrant pursuant to this Section 7.

               (e) Warrant Register. The Company shall maintain, at its
principal executive offices (or such other office or agency of the Company as it
may designate by notice to the holder hereof), a register for this Warrant, in
which the Company shall record the name and address of the person in whose name
this Warrant has been issued, as well as the name and address of each transferee
and each prior owner of this Warrant.

               (f) Exercise or Transfer Without Registration. If, at the time of
the surrender of this Warrant in connection with any exercise, transfer, or
exchange of this Warrant, this Warrant (or, in the case of any exercise, the
Warrant Shares issuable hereunder), shall not be registered under the Securities
Act and under applicable state securities or blue sky laws, the Company may
require, as a condition of allowing such exercise, transfer, or exchange, (i)
that the holder or transferee of this Warrant, as the case may be, furnish to
the Company a written opinion of counsel (which opinion shall be in form,
substance and scope customary for opinions of counsel in comparable
transactions) to the effect that such exercise, transfer, or exchange may be
made without registration under the Securities Act and under applicable state
securities or blue sky laws (the cost of which shall be borne by the Company if
the Company's counsel renders such an opinion and up to $500 of such cost shall
be borne by the Company if the holder's counsel is requested to render such
opinion), (ii) that the holder or transferee execute and deliver to the Company
an investment letter in form and substance acceptable to the Company and (iii)
that the transferee be an "ACCREDITED INVESTOR" as defined in Rule 501(a)
promulgated under the Securities Act; provided that no such opinion, letter, or
status as an "accredited investor" shall be required in connection with a
transfer pursuant to Rule 144 under the Securities Act.

               (g) Additional Restrictions on Exercise or Transfer. In no event
shall the holder hereof have the right to exercise any portion of this Warrant
for shares of Common Stock or to dispose of any portion of this Warrant to the
extent that such right to effect such exercise or disposition would result in
the holder or any of its affiliates together beneficially owning more than (i)
19.99% of the outstanding shares of Common Stock, in the case of Biotech Target
N.V. and its affiliates for so long as they continue to hold Warrants, or (b)
4.99% of the outstanding shares of Common Stock, in the case of all other
holders of Warrants. For purposes of this Section 7(g), beneficial ownership
shall be determined in accordance with Section 13(d) of the Securities Exchange
Act of 1934, as amended, and Regulation 13D-G thereunder. The restriction
contained in this Section 7(g) may not be altered, amended, deleted or changed
in any manner whatsoever unless the holders of a majority of the outstanding
shares of Common Stock and the holder hereof shall approve, in writing, such
alteration, amendment, deletion or change.

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        8. Registration Rights. The initial holder of this Warrant (and certain
assignees thereof) is entitled to the benefit of such registration rights in
respect of the Warrant Shares as are set forth in the Registration Rights
Agreement, including the right to assign such rights to certain assignees, as
set forth therein.

        9. Notices. Any notices required or permitted to be given under the
terms of this Warrant shall be sent by certified or registered mail (return
receipt requested) or delivered personally or by courier or by confirmed
telecopy, and shall be effective five days after being placed in the mail, if
mailed, or upon receipt or refusal of receipt, if delivered personally or by
courier, or by confirmed telecopy, in each case addressed to a party. The
addresses for such communications shall be:

                      If to the Company:

                      ViroLogic, Inc.
                      270 East Grand Avenue
                      South San Francisco, California 94080
                      Telephone:  (650) 635-1100
                      Attn:  Chief Executive Officer

                      with a copy simultaneously transmitted by like means to:

                      Cooley Godward LLP
                      4401 Eastgate Mall
                      San Diego, California 92121
                      Telephone:  (858) 550-6000

If to the holder, at such address as such holder shall have provided in writing
to the Company, or at such other address as such holder furnishes by notice
given in accordance with this Section 9.

        10. Governing Law; Jurisdiction. This Warrant shall be governed by and
construed in accordance with the laws of the State of Delaware. The Company
irrevocably consents to the jurisdiction of the United States federal courts and
state courts located in the State of Delaware in any suit or proceeding based on
or arising under this Warrant and irrevocably agrees that all claims in respect
of such suit or proceeding may be determined in such courts. The Company
irrevocably waives any objection to the laying of venue and the defense of an
inconvenient forum to the maintenance of such suit or proceeding. The Company
further agrees that service of process upon the Company mailed by certified or
registered mail shall be deemed in every respect effective service of process
upon the Company in any such suit or proceeding. Nothing herein shall affect the
holder's right to serve process in any other manner permitted by law. The
Company agrees that a final non-appealable judgment in any such suit or
proceeding shall be conclusive and may be enforced in other jurisdictions by
suit on such judgment or in any other lawful manner.

                                       10
<PAGE>

        11. Miscellaneous.

               (a) Amendments. Except as provided in Section 7(g) hereof, this
Warrant and any provision hereof may only be amended by an instrument in writing
signed by the Company and the holder hereof.

               (b) Descriptive Headings. The descriptive headings of the several
Sections of this Warrant are inserted for purposes of reference only, and shall
not affect the meaning or construction of any of the provisions hereof.

               (c) Cashless Exercise. This Warrant may be exercised at any time
after the first anniversary of the Issue Date and before the expiration of the
Exercise Period by presentation and surrender of this Warrant to the Company at
its principal executive offices with a written notice of the holder's intention
to effect a cashless exercise, including a calculation of the number of shares
of Common Stock to be issued upon such exercise in accordance with the terms
hereof (a "CASHLESS EXERCISE"). In the event of a Cashless Exercise, in lieu of
paying the Exercise Price in cash, the holder shall surrender this Warrant for
that number of shares of Common Stock determined by multiplying the number of
Warrant Shares to which it would otherwise be entitled by a fraction, the
numerator of which shall be the difference between the then current Market Price
of a share of the Common Stock on the date of exercise and the Exercise Price,
and the denominator of which shall be the then current Market Price per share of
Common Stock.

               (d) Trading Day. For purposes of this Warrant, the term "trading
day" means any day on which the principal United States securities exchange or
trading market where the Common Stock is then listed is open for trading.

               (e) Redemption of Warrant by Company.

                      (i) In the event that the Company consummates a Corporate
Change in which the holders of the Company's Common Stock immediately prior to
such Corporate Change hold fifty percent (50%) or less of the outstanding voting
power of the entity surviving such Corporate Change, then the Company may elect,
upon delivery of at least twenty (20) days' prior written notice (the
"REDEMPTION NOTICE") to the holder hereof, to redeem all or a portion of the
Warrant for a redemption amount (the "REDEMPTION AMOUNT") equal to the value of
the Warrant derived using the Black-Scholes formula (using Bloomberg), assuming
that the volatility of the Company's Common Stock equals 60% and the risk-free
interest rate equals 5% per annum. In the event the Company elects to redeem
only some of the outstanding Warrants pursuant to this Section 11(e), such
Warrants shall be redeemed pro rata among all the holders of the Warrants based
upon the percentage of Warrants held by such holders against the total
outstanding Warrants.

                      (ii) [Intentionally Omitted]

                      (iii) The Company may not deliver a Redemption Notice
pursuant to clause (i) above unless on or prior to the date of delivery of a
Redemption Notice, the Company shall have segregated on the books and records of
the Company an amount of cash sufficient to pay all amounts to which holders of
the Warrants that are being redeemed are entitled pursuant to this Section
11(e).

                                       11
<PAGE>

Any Redemption Notice delivered shall be irrevocable and shall be accompanied by
a statement executed by a duly authorized officer of the Company.

                      (iv) The Redemption Amount shall be paid to the holder
within three (3) business days of the date of redemption set forth in the
Redemption Notice pursuant to clause (i) above; provided, however, that the
Company shall not be obligated to deliver any portion of the Redemption Amount
until either this Warrant is delivered to the Company or the holder notifies the
Company that the Warrant has been lost, stolen or destroyed and delivers the
documentation in accordance with Section 7(c) hereof. In the event only a
portion of this Warrant is being redeemed, the Company shall issue, at its
expense, a new Warrant representing the number of shares with respect to which
this Warrant shall not then have been redeemed or exercised.

                      (v) Notwithstanding the delivery of a Redemption Notice
pursuant to clause (i) above, the holder may exercise all or a portion of this
Warrant subject to such Redemption Notice by the delivery prior to the date of
redemption set forth in such notice of an Exercise Agreement pursuant to the
procedures set forth in Section 1.

               (f) Indemnification by Company.

                      (i) The Company shall hold harmless and indemnify the
holder of this Warrant from and against, and shall compensate and reimburse such
holder for, any damages which are directly or indirectly suffered or incurred by
such holder or to which such holder may otherwise become subject (regardless of
whether or not such damages relate to any third-party claim) and which arise
from or as a result of, or are directly or indirectly connected with any breach
of any of the Company's covenants set forth herein.

                      (ii) In the event of the assertion or commencement by any
person of any claim or legal proceeding with respect to which the holder may
have indemnification rights pursuant to this Section 11(f)(i), the holder shall
promptly notify the Company thereof in writing, but the failure to so notify the
Company will not limit the holder's rights to indemnification hereunder, except
to the extent the Company demonstrates that the defense of such action is
prejudiced by the failure to so give such notice.

                                       12
<PAGE>

        IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by
its duly authorized officer.

                                        VIROLOGIC, INC.

                                        By:
                                            ------------------------------------
                                            Name:
                                                 -------------------------------
                                            Title:
                                                  ------------------------------

                                       13
<PAGE>

                           FORM OF EXERCISE AGREEMENT

                (TO BE EXECUTED BY THE HOLDER IN ORDER TO EXERCISE THE WARRANT)

To:     ViroLogic,Inc.
        270 East Grand Avenue
        South San Francisco, California 94080
        Telephone:  (650) 635-1100
        Attn:  Chief Executive Officer

        The undersigned hereby irrevocably exercises the right to purchase
_____________ shares of the Common Stock of ViroLogic, Inc., a corporation
organized under the laws of the State of Delaware (the "COMPANY"), evidenced by
the attached Warrant, and herewith [makes payment of the Exercise Price with
respect to such shares in full][elects to effect a Cashless Exercise (as defined
in Section 11(c) of such Warrant)], all in accordance with the conditions and
provisions of said Warrant.

        The undersigned agrees not to offer, sell, transfer or otherwise dispose
of any Common Stock obtained on exercise of the Warrant, except under
circumstances that will not result in a violation of the Securities Act of 1933,
as amended, or any state securities laws. The undersigned represents that it is
an "accredited investor" as that term is defined in Rule 501(a) of Regulation D
under the Securities Act of 1933, as amended.

[ ]     The undersigned requests that the Company cause its transfer agent to
        electronically transmit the Common Stock issuable pursuant to this
        Exercise Agreement to the account of the undersigned or its nominee
        (which is _________________) with DTC through its Deposit Withdrawal
        Agent Commission System ("DTC TRANSFER"), provided that such transfer
        agent participates in the DTC Fast Automated Securities Transfer
        program.

[ ]     In lieu of receiving the shares of Common Stock issuable pursuant to
        this Exercise Agreement by way of DTC Transfer, the undersigned hereby
        requests that the Company cause its transfer agent to issue and deliver
        to the undersigned physical certificates representing such shares of
        Common Stock.

        The undersigned requests that a Warrant representing any unexercised
portion hereof be issued, pursuant to the Warrant, in the name of the Holder and
delivered to the undersigned at the address set forth below:

Dated:
      -----------------                 ----------------------------------------
                                              Signature of Holder

                                        ----------------------------------------
                                              Name of Holder (Print)

                                              Address:

                                        ----------------------------------------

                                        ----------------------------------------

                                        ----------------------------------------

                                       14
<PAGE>

                               FORM OF ASSIGNMENT

        FOR VALUE RECEIVED, the undersigned hereby sells, assigns, and transfers
all the rights of the undersigned under the within Warrant, with respect to the
number of shares of Common Stock covered thereby set forth hereinbelow, to:

Name of Assignee                    Address                       No. of Shares
----------------                    -------                       -------------

, and hereby irrevocably constitutes and appoints
_____________________________________ as agent and attorney-in-fact to transfer
said Warrant on the books of the within-named corporation, with full power of
substitution in the premises.

Dated:                      ,
       ---------------------  ----

In the presence of

------------------

                                   Name:
                                         ---------------------------------------

                                         Signature:
                                                    ----------------------------
                                         Title of Signing Officer or Agent (if
                                         any):
                                                    ----------------------------

                                         Address:
                                                    ----------------------------

                                                    ----------------------------

                                         Note:  The above signature should
                                                correspond exactly with the name
                                                on the face of the within
                                                Warrant.

                                       15<PAGE>
                                                                     EXHIBIT 4.2

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER
THE SECURITIES ACT OF 1933, AS AMENDED, OR THE SECURITIES LAWS OF ANY STATE OF
THE UNITED STATES OR IN ANY OTHER JURISDICTION. THE SECURITIES REPRESENTED
HEREBY MAY NOT BE OFFERED, SOLD OR TRANSFERRED IN THE ABSENCE OF AN EFFECTIVE
REGISTRATION STATEMENT FOR THE SECURITIES UNDER APPLICABLE SECURITIES LAWS
UNLESS OFFERED, SOLD OR TRANSFERRED PURSUANT TO AN AVAILABLE EXEMPTION FROM THE
REGISTRATION REQUIREMENTS OF THOSE LAWS.

                  SERIES C CONVERTIBLE SECURED PROMISSORY NOTE

Date: November 19, 2002                                               $_________

               FOR VALUE RECEIVED, VIROLOGIC, INC., a corporation organized
under the laws of the State of Delaware (hereinafter called the "BORROWER" or
the "CORPORATION"), hereby promises to pay to the order of ____________________,
a ____________________, or registered assigns (individually, the "HOLDER", and
collectively with the holders of all other notes of same like and tenor, the
"HOLDERS"), the sum of _______________________($_________) on March 19, 2003
(the "SCHEDULED MATURITY DATE"), and to pay interest on the unpaid principal
balance hereof at the rate of eight percent (8%) per annum. The principal amount
hereof, together with all accrued and unpaid interest thereon, shall be due and
payable on the Scheduled Maturity Date. Interest shall accrue on the unpaid
principal balance hereof from the date hereof (the "ISSUE DATE") until the same
becomes due and payable, whether at maturity, or upon prepayment, repayment, or
otherwise. Interest shall be calculated based on a 365 day year and shall be
payable in arrears. All payments of principal (to the extent not converted in
accordance with the terms hereof) and interest shall be made in, and all
references herein to monetary denominations shall refer to, lawful money of the
United States of America. All payments shall be made at such address as the
Holder shall have given or shall hereafter give to the Borrower by written
notice made in accordance with the provisions of this Note.

        The term "NOTE" and all references thereto, as used throughout this
instrument, shall mean this instrument as originally executed, or if later
amended or supplemented, then as so amended or supplemented. This Note is being
issued by the Borrower along with similar convertible notes designated as Series
C Convertible Secured Promissory Notes (the "OTHER NOTES" and, together with
this Note, the "NOTES") pursuant to that certain Exchange Agreement, dated as of
the date hereof, between the Borrower and the signatories thereto (the "EXCHANGE
AGREEMENT"). The obligations under the Notes are secured as provided in a
Security Agreement, dated as of the date hereof, by the Borrower in favor of SDS
Merchant Fund, L.P., as collateral agent for the benefit of the Holders of the
Notes (the "SECURITY AGREEMENT") and in the Intellectual Property Security
Agreement, dated as of the date hereof, by the Borrower in favor of SDS Merchant
Fund, L.P., as collateral agent for the benefit of the Holders of the Notes (the

<PAGE>

"IP SECURITY AGREEMENT"). The Notes, the Exchange Agreement, the Security
Agreement, the IP Security Agreement and the Registration Rights Agreement,
dated as of the date hereof, between the Corporation and the parties thereto
(the "REGISTRATION RIGHTS AGREEMENT") are collectively referred to herein as the
"TRANSACTION DOCUMENTS."

                                    ARTICLE I

                                   PREPAYMENT

        A. Mandatory Prepayment. Upon the occurrence of an Event of Default (as
defined below), this Note shall be prepaid by the Borrower in accordance with
the provisions of Article V hereof.

        B. Prepayment at Borrower's Option. This Note may not be prepaid at the
option of Borrower without the prior written consent of the Holder.

                                   ARTICLE II

                               CERTAIN DEFINITIONS

        The following terms shall have the following meanings:

        A. "CONVERSION AMOUNT" means the outstanding principal amount of this
Note (but none of the accrued and unpaid interest thereon) on the Conversion
Date.

        B. "CONVERSION DATE" means the date of the Automatic Conversion Trigger
Event.

        C. "CONVERSION SHARES" means the shares of Series C Preferred Stock
issuable upon conversion of this Note.

        D. "CONVERSION PRICE" means $10,000 per share of Series C Preferred
Stock, subject to adjustment as set forth herein.

        E. "EXCESS INTEREST PAYMENT AMOUNT" means an amount equal to the amount
by which the Interest Payment Amount exceeds the Premium Payment Amount.

        F. "INTEREST PAYMENT AMOUNT" an amount equal to the amount of all
accrued and unpaid interest due on the principal amount of this Note on the
Conversion Date.

        G. "PREMIUM PAYMENT AMOUNT" means an amount equal to the amount of all
accrued and unpaid Premium (as such term is defined in the Series C Certificate
of Designations) due on the Conversion Shares on the Conversion Date.

                                     - 2 -
<PAGE>

        H. "SERIES C CERTIFICATE OF DESIGNATIONS" means the Corporation's
Certificate of Designation, Preferences and Rights of Series C Preferred Stock,
as in effect from time to time.

        I. "SERIES C PREFERRED STOCK" means shares of the Corporation's Series C
Convertible Preferred Stock, par value $.001 per share.

                                   ARTICLE III

                                   CONVERSION

        A. Mandatory Conversion. If each of the Authorized Stock Approval (as
such term is defined in the Exchange Agreement) and the NASD Rule Approval (as
such term is defined in the Exchange Agreement) is obtained on or prior to the
Scheduled Maturity Date (the "AUTOMATIC CONVERSION TRIGGER EVENT"), then (i) the
principal amount of each of the Notes (but none of the accrued and unpaid
interest due thereon), shall be automatically converted into a number of fully
paid and nonassessable shares of Series C Preferred Stock (determined in
accordance with the formula set forth in Article III.B) and (ii) (A) the Excess
Interest Payment Amount shall immediately become due and payable and (B) all
other accrued and unpaid interest due on the principal amount of each of the
Notes shall be forgiven. The Corporation shall provide immediate written notice
to each Holder of an Automatic Conversion Trigger Event and the Corporation and
the Holders shall follow the applicable conversion procedures set forth in
Article III.C.

        B. Conversion Formula. The number of shares of Series C Preferred Stock
issuable upon payment of the Conversion Price shall be determined in accordance
with the following formula:

                                CONVERSION AMOUNT
                                CONVERSION PRICE

        C. Mechanics of Conversion. Upon receipt by the Holder of written notice
of an Automatic Conversion Trigger Event, the Holder shall surrender or cause to
be surrendered this Note, duly endorsed, as soon as practicable thereafter to
the Corporation or the transfer agent. Upon receipt by the Corporation of a
facsimile copy of this Note from the Holder, the Corporation shall immediately
send, via facsimile, a confirmation to the Holder stating that this Note has
been received, the date upon which the Corporation expects to deliver the Series
C Preferred Stock issuable upon such conversion and the name and telephone
number of a contact person at the Corporation regarding the conversion. The
Corporation shall not be obligated to issue shares of Series C Preferred Stock
upon a conversion unless either this Note is delivered to the Corporation or the
transfer agent as provided above, or the Holder notifies the Corporation or the
transfer agent that such certificates have been lost, stolen or destroyed and
delivers the documentation to the Corporation required by Article IX.H hereof.

               (i) Delivery of Series C Preferred Stock Upon Conversion. Upon
the surrender of this Note pursuant to Paragraph C above, the Corporation shall,
no later than the

                                     - 3 -
<PAGE>

later of (a) the second business day following the Conversion Date and (b) the
business day following the date of such surrender (or, in the case of lost,
stolen or destroyed certificates, after provision of indemnity pursuant to
Article IX.H), issue and deliver to the Holder or its nominee the number of
shares of Series C Preferred Stock issuable upon conversion of the Note. The
Corporation shall deliver to the Holder physical certificates representing the
Series C Preferred Stock issuable upon conversion.

               (ii) Taxes. The Corporation shall pay any and all taxes which may
be imposed upon it with respect to the issuance and delivery of the shares of
Series C Preferred Stock upon the conversion of this Note.

               (iii) Fractional Shares. If any conversion of this Note would
result in the issuance of a fractional share of Series C Preferred Stock, the
Corporation shall issue such fractional share upon such conversion.

                                   ARTICLE IV

                RESERVATION OF SHARES OF SERIES C PREFERRED STOCK

        A. Reserved Amount. On the Issue Date, the Corporation shall reserve
1205 shares of the authorized but unissued shares of Series C Preferred Stock
for issuance upon conversion of each of the Notes pursuant to Article III.A
thereof and thereafter the number of authorized but unissued shares of Series C
Preferred Stock so reserved shall not be decreased and shall at all times be
sufficient to provide for the conversion of each of the Notes pursuant to
Article III.A thereof at the then current Conversion Price thereof.

                                    ARTICLE V

                                EVENTS OF DEFAULT

        A. Events of Default. In the event (each of the events described in
clauses (i)-(vii) below after expiration of the applicable cure period (if any)
being an "EVENT OF DEFAULT"):

               (i) the Corporation fails to pay the principal hereof, and/or the
accrued and unpaid interest thereon, when due, whether at maturity, upon
acceleration or otherwise;

               (ii) except with respect to matters covered by subparagraph (i)
above, as to which such subparagraph shall apply, the Corporation otherwise
shall breach any material term hereunder or under the other Transaction
Documents, including, without limitation, the representations and warranties
contained therein (i.e., in the event of a material breach as of the date such
representation and warranty was made) and if such breach is curable, shall fail
to cure such breach within 10 business days after the Corporation has been
notified thereof in writing by the Holder;

               (iii)  the Corporation shall:

                                     - 4 -
<PAGE>

                      (a) sell, convey or dispose of all or substantially all of
its assets (the presentation of any such transaction for stockholder approval
being conclusive evidence that such transaction involves the sale of all or
substantially all of the assets of the Corporation);

                      (b) merge, consolidate or engage in any other business
combination with any other entity (other than pursuant to a migratory merger
effected solely for the purpose of changing the jurisdiction of incorporation of
the Corporation and other than pursuant to a merger in which the Corporation is
the surviving or continuing entity and its capital stock is unchanged) provided
that such merger, consolidation or business combination is required to be
reported by the Corporation on a Current Report pursuant to Item 1 of Form 8-K,
or any successor form;

                      (c) either (i) fail to pay, when due, or within any
applicable grace period, any payment with respect to any indebtedness of the
Corporation in excess of $350,000 due to any third party, other than payments
contested by the Corporation in good faith, or otherwise breach or violate any
agreement for monies owed or owing in an amount in excess of $350,000 which
breach or violation permits the other party thereto to declare a default or
otherwise accelerate amounts due thereunder, or (ii) suffer to exist any other
default or event of default under any agreement binding the Corporation which
default or event of default would or is likely to have a material adverse effect
on the business, operations, properties, prospects or financial condition of the
Corporation;

                      (d) on or prior to the second anniversary of the Issue
Date, issue or agree to issue any future equity or equity-linked securities or
debt which is convertible into equity or in which there is an equity component,
except for any Excluded Issuance (as such term is defined in the Series C
Certificate of Designations);

               (iv) the Corporation or any subsidiary of the Corporation shall
make an assignment for the benefit of creditors, or apply for or consent to the
appointment of a receiver or trustee for it or for a substantial part of its
property or business; or such a receiver or trustee shall otherwise be
appointed;

               (v) bankruptcy, insolvency, reorganization or liquidation
proceedings or other proceedings for relief under any bankruptcy law or any law
for the relief of debtors shall be instituted by or against the Corporation or
any subsidiary of the Corporation, and if instituted against the Corporation or
any subsidiary of the Corporation by a third party, shall not be dismissed
within 60 days of their initiation, or

               (vi) a Redemption Event (as such term is defined in the Series C
Certificate of Designations) shall occur and be continuing under the Series C
Certificate of Designations,

               (vii) an Event of Default (as such term is defined in the
applicable Transaction Document) shall occur and be continuing under any other
Transaction Document,

then, upon the occurrence of any such Event of Default, at the option of each
Holder, exercisable in whole or in part at any time and from time to time by
delivery of a Default Notice (as defined in Paragraph C below) to the
Corporation while such Event of Default continues, the Corporation

                                     - 5 -
<PAGE>

shall pay the Holders (and upon the occurrence of an Event of Default specified
in subparagraphs (iv) and (v) of this Paragraph A, the Corporation shall be
required to pay the Holders), in satisfaction of its obligation to pay the
outstanding principal amount of the Notes and accrued and unpaid interest
thereon, an amount equal to the Default Amount (as defined in Paragraph B below)
and such Default Amount, together with all other ancillary amounts payable
hereunder, shall immediately become due and payable, all without demand,
presentment or notice, all of which are hereby expressly waived, together with
all costs, including, without limitation, legal fees and expenses of collection,
and the Holder shall be entitled to exercise all other rights and remedies
available at law or in equity. For the avoidance of doubt, the occurrence of any
event described in clauses (i), (iii), (iv), (v), (vi) and (vii) above shall
immediately constitute an Event of Default and there shall be no cure period.
Upon the Corporation's receipt of any Default Notice hereunder, the Corporation
shall immediately (and in any event within one business day following such
receipt) deliver a written notice (a "DEFAULT ANNOUNCEMENT") to all Holders of
the Notes stating the date upon which the Corporation received such Default
Notice and the amount of the Notes covered thereby. Following the delivery of a
Default Announcement hereunder, at any time and from time to time, each Holder
of the Notes may request (either orally or in writing) information from the
Corporation with respect to the instant default (including, but not limited to,
the aggregate principal amount outstanding of Notes covered by Default Notices
received by the Corporation) and the Corporation shall furnish (either orally or
in writing) as soon as practicable such requested information to such requesting
Holder.

        B. Definition of Default Amount. The "DEFAULT AMOUNT" with respect to a
Note means an amount equal to the aggregate principal amount outstanding of the
Notes being paid plus all accrued and unpaid interest thereon through the
payment date.

        C. Failure to Pay Default Amounts. If the Corporation fails to pay any
Holder the Default Amount with respect to any Note within five business days
after its receipt of a notice requiring such repayment (a "DEFAULT NOTICE"),
then the Holder of any Note delivering such Default Notice shall be entitled to
interest on the Default Amount at a per annum rate equal to the lower of
twenty-four percent (24%) and the highest interest rate permitted by applicable
law from the date on which the Corporation receives the Default Notice until the
date of payment of the Default Amount hereunder. In the event the Corporation is
not able to repay all of the outstanding Notes subject to Default Notices
delivered prior to the date upon which such repayment is to be effected, the
Corporation shall repay the outstanding Notes from each Holder pro rata, based
on the total amounts due on the Notes at the time of repayment included by such
Holder in all Default Notices delivered prior to the date upon which such
repayment is to be effected relative to the total amounts due under the Notes at
the time of repayment included in all of the Default Notices delivered prior to
the date upon which such repayment is to be effected.

                                   ARTICLE VI

                       ADJUSTMENTS TO THE CONVERSION PRICE

        The Conversion Price shall be subject to adjustment from time to time as
follows:

                                     - 6 -
<PAGE>

        A. Stock Splits, Stock Dividends, Etc. If, at any time on or after the
Issue Date, the number of outstanding shares of Series C Preferred Stock is
increased by a stock split, stock dividend, combination, reclassification or
other similar event, the Conversion Price shall be proportionately reduced, or
if the number of outstanding shares of Series C Preferred Stock is decreased by
a reverse stock split, combination or reclassification of shares, or other
similar event, the Conversion Price shall be proportionately increased. In such
event, the Corporation shall notify the Corporation's transfer agent of such
change on or before the effective date thereof.

        B. Adjustment Due to Merger, Consolidation, Etc. If, at any time after
the Issue Date, there shall be (i) any reclassification or change of the
outstanding shares of Series C Preferred Stock (other than a change in par
value, or from par value to no par value, or from no par value to par value, or
as a result of a subdivision or combination), (ii) any consolidation or merger
of the Corporation with any other entity (other than a merger in which the
Corporation is the surviving or continuing entity and its capital stock is
unchanged), (iii) any sale or transfer of all or substantially all of the assets
of the Corporation or (iv) any share exchange pursuant to which all of the
outstanding shares of Series C Preferred Stock are converted into other
securities or property (each of (i) - (iv) above being a "CORPORATE CHANGE"),
then appropriate provisions (in form and substance reasonably satisfactory to
the Holders of a majority of the principal amount of the Notes then outstanding)
shall be made with respect to the rights and interests of the Holders of the
Notes to the end that the economic value of the Notes are in no way diminished
by such Corporate Change and that the provisions hereof (including, without
limitation, in the case of any such consolidation, merger or sale in which the
successor entity or purchasing entity is not the Corporation, an immediate
adjustment of the Conversion Price so that the Conversion Price immediately
after the Corporate Change reflects the same relative value as compared to the
value of the surviving entity's common stock that existed between the Conversion
Price and the value of the Corporation's common stock immediately prior to such
Corporate Change) shall thereafter be applicable, as nearly as may be
practicable in relation to any shares of stock or securities thereafter
deliverable upon the conversion thereof. The Corporation shall not effect any
Corporate Change unless (i) each Holder of the Notes has received written notice
of such transaction at least 45 days prior thereto, but in no event later than
15 days prior to the record date for the determination of shareholders entitled
to vote with respect thereto, (ii) the resulting successor or acquiring entity
(if not the Corporation) assumes by written instrument (in form and substance
reasonably satisfactory to the Holders of a majority of the principal amount of
the Notes then outstanding) the obligations of the Notes and (iii) the
Corporation shall have received the prior written consent of the Holders of a
majority of the principal amount of the Notes then outstanding. The above
provisions shall apply regardless of whether or not there would have been a
sufficient number of shares of Series C Preferred Stock authorized and available
for issuance upon conversion of the Notes outstanding as of the date of such
transaction, and shall similarly apply to successive reclassifications,
consolidations, mergers, sales, transfers or share exchanges.

        C. Adjustment Due to Distribution. If, at any time after the Issue Date,
the Corporation shall declare or make any distribution of its assets (or rights
to acquire its assets) to holders of Series C Preferred Stock as a partial
liquidating dividend, by way of return of capital or otherwise (including any
dividend or distribution to the Corporation's shareholders in cash or

                                     - 7 -
<PAGE>

shares (or rights to acquire shares) of capital stock of a subsidiary (i.e. a
spin-off)) (a "DISTRIBUTION"), then the Holders of the Notes shall be entitled,
upon any conversion of the Notes after the date of record for determining
shareholders entitled to such Distribution, to receive the amount of such assets
which would have been payable to the Holder with respect to the shares of Series
C Preferred Stock issuable upon such conversion had such Holder been the holder
of such shares of Series C Preferred Stock on the record date for the
determination of shareholders entitled to such Distribution.

        D. Purchase Rights. If, at any time after the Issue Date, the
Corporation issues any warrants, rights or options, whether or not immediately
exercisable, to subscribe for or to purchase Series C Preferred Stock or
securities exercisable, convertible into or exchangeable for Series C Preferred
Stock or rights to purchase stock, warrants, securities or other property (the
"PURCHASE RIGHTS") pro rata to the record holders of Series C Preferred Stock,
then the Holders of the Notes will be entitled to acquire, upon the terms
applicable to such Purchase Rights, the aggregate Purchase Rights which such
Holder could have acquired if such Holder had held the number of shares of
Series C Preferred Stock acquirable upon complete conversion of the Notes
immediately before the date on which a record is taken for the grant, issuance
or sale of such Purchase Rights, or, if no such record is taken, the date as of
which the record holders of Series C Preferred Stock are to be determined for
the grant, issue or sale of such Purchase Rights.

        E. Notice of Adjustments. Upon the occurrence of each adjustment or
readjustment of the Conversion Price pursuant to this Article VI amounting to a
more than 5% change in such Conversion Price, the Corporation, at its expense,
shall promptly compute such adjustment or readjustment and prepare and furnish
to each Holder a certificate setting forth such adjustment or readjustment and
showing in detail the facts upon which such adjustment or readjustment is based.
The Corporation shall, upon the written request at any time of any Holder of the
Notes, furnish to such Holder a like certificate setting forth (i) such
adjustment or readjustment, (ii) the Conversion Price at the time in effect and
(iii) the number of shares of Series C Preferred Stock and the amount, if any,
of other securities or property which at the time would be received upon
conversion of any Note.

        F. Other Action Affecting Conversion Price. If the Corporation takes any
action affecting the Series C Preferred Stock after the date hereof that would
be covered by Article VI.A through D, but for the manner in which such action is
taken or structured, which would in any way diminish the value of the Notes,
then the Conversion Price shall be adjusted in such manner as the Board of
Directors of the Corporation shall in good faith determine to be equitable under
the circumstances.

                                   ARTICLE VII

                                 CONSENT RIGHTS

        A. So long as any Notes are outstanding, except as otherwise expressly
provided in the Exchange Agreement, the Corporation shall not, in each case
without first obtaining the

                                     - 8 -
<PAGE>

written consent of the Holders of a majority of the then outstanding principal
amount of the Notes: (i)(a) redeem, or declare or pay any dividends (whether in
cash or stock), or otherwise make any distributions with respect to any class or
series of capital stock of the Corporation, except for dividends and
distributions payable solely in the capital stock of the Corporation, or (b)
prepay any outstanding indebtedness of the Corporation, or (ii) create or sell
any securities that rank senior to or pari passu with the Notes.

                                  ARTICLE VIII

                                  MISCELLANEOUS

        A. Failure or Indulgency Not Waiver. No failure or delay on the part of
any Holder in the exercise of any power, right or privilege hereunder shall
operate as a waiver thereof, nor shall any single or partial exercise of any
such power, right or privilege preclude other or further exercise thereof or of
any other right, power or privilege.

        B. Notices. Any notices required or permitted to be given under the
terms of this Note shall be sent by certified or registered mail (return receipt
requested) or delivered personally or by courier or by confirmed telecopy, and
shall be effective five days after being placed in the mail, if mailed, or upon
receipt or refusal of receipt, if delivered personally or by courier or
confirmed telecopy, in each case addressed to a party. The addresses for such
communications shall be:

                      If to the Corporation:

                      ViroLogic, Inc.
                      270 East Grand Avenue
                      South San Francisco, California 94080
                      Telephone:  (650) 635-1100
                      Attn: Chief Executive Officer

                      with a copy simultaneously transmitted by like means to
                      (which transmittal shall not constitute notice hereunder):

                      Cooley Godward LLP
                      4401 Eastgate Mall
                      San Diego, California 92121
                      Telephone:  (858) 550-6000

        If to the Holder, to the address set forth under such Holder's name on
the signature page to the Exchange Agreement executed by such Holder. Each party
shall provide notice to the other parties of any change in address or the
address of any transferee of the Note.

                                     - 9 -
<PAGE>

        C. Amendment Provision. This Note and any provision hereof may be
amended only by an instrument in writing signed by the Corporation and the
Holders of a majority of the then outstanding principal amount of the Notes.

        D. Assignability. This Note shall be binding upon the Corporation and
its successors and assigns and shall inure to the benefit of the Holder and its
successors and assigns. Notwithstanding anything to the contrary contained in
this Note or the Transaction Documents, this Note may be pledged and all rights
of the Holder under this Note may be assigned to any affiliate or to any other
person or entity without the consent of the Corporation.

        E. Cost of Collection. If an Event of Default occurs hereunder, the
Corporation shall pay the Holder hereof costs of collection, including
reasonable attorneys' fees.

        F. Governing Law; Jurisdiction. This Note shall be governed by and
construed in accordance with the laws of the State of Delaware applicable to
contracts made and to be performed in the State of Delaware. The Corporation
irrevocably consents to the jurisdiction of the United States federal courts and
the state courts located in the State of Delaware in any suit or proceeding
based on or arising under this Note and irrevocably agrees that all claims in
respect of such suit or proceeding may be determined in such courts. The
Corporation irrevocably waives the defense of an inconvenient forum to the
maintenance of such suit or proceeding. The Corporation further agrees that
service of process upon the Corporation mailed by first class mail shall be
deemed in every respect effective service of process upon the Corporation in any
such suit or proceeding. Nothing herein shall affect the right of any Holder to
serve process in any other manner permitted by law. The Corporation agrees that
a final non-appealable judgment in any such suit or proceeding shall be
conclusive and may be enforced in other jurisdictions by suit on such judgment
or in any other lawful manner.

        G. Denominations. At the request of the Holder, upon surrender of this
Note, the Corporation shall promptly issue new Notes in the aggregate
outstanding principal amount hereof, in the form hereof, in such denominations
of at least $25,000 as the Holder shall request.

        H. Lost or Stolen Notes. Upon receipt by the Corporation of (i) evidence
of the loss, theft, destruction or mutilation of any Note and (ii) (y) in the
case of loss, theft or destruction, of indemnity (without any bond or other
security) reasonably satisfactory to the Corporation, or (z) in the case of
mutilation, upon surrender and cancellation of any Note, the Corporation shall
execute and deliver a new Note of like tenor and date.

        I. Payment of Cash; Defaults. Whenever the Corporation is required to
make any cash payment to a Holder under the Notes (whether a Default Amount or
upon prepayment, repayment or otherwise), such cash payment shall be made in
U.S. dollars to the Holder within five business days after delivery by such
Holder of a notice specifying that the Holder elects to receive such payment in
cash and the method (e.g., by check, wire transfer) in which such payment should
be made. If such payment is not delivered within such five business day period,
such Holder shall thereafter be entitled to interest on the unpaid amount at a
per annum rate equal to the lower of twenty-four percent (24%) and the highest
interest rate permitted by applicable law until such amount is paid in full to
the Holder.

                                     - 10 -
<PAGE>

        J. Status as Note Holder. Upon an Automatic Conversion Trigger Event,
(i) the principal amount of the Notes (but none of the accrued and unpaid
interest thereon) shall be deemed converted into shares of Series C Preferred
Stock as of the Conversion Date and (ii) the Holder's rights as a Holder of such
Notes shall cease and terminate, excepting only the right (A) to receive
certificates for such shares of Series C Preferred Stock, (B) to receive payment
of the Excess Interest Payment Amount, and (C) to exercise any remedies provided
herein or otherwise available at law or in equity to such Holder because of a
failure by the Corporation to comply with the terms of the Notes.

        K. Remedies Cumulative. The remedies provided in this Note shall be
cumulative and in addition to all other remedies available under this Note, at
law or in equity (including a decree of specific performance and/or other
injunctive relief), and nothing herein shall limit a Holder's right to pursue
actual damages for any failure by the Corporation to comply with the terms of
this Note. The Corporation acknowledges that a breach by it of its obligations
hereunder will cause irreparable harm to the Holders of the Notes and that the
remedy at law for any such breach may be inadequate. The Corporation therefore
agrees, in the event of any such breach or threatened breach, that the Holders
of the Notes shall be entitled, in addition to all other available remedies, to
an injunction restraining any breach, without the necessity of showing economic
loss and without any bond or other security being required.

        L. Business Day. For purposes of this Note, the term "BUSINESS DAY"
means any day, other than a Saturday or Sunday or a day on which banking
institutions in the State of Delaware are authorized or obligated by law,
regulation or executive order to close. If any payment to be made hereunder
shall be stated to be or become due on a day which is not a business day, such
payment shall be made on the next following business day and such extension of
time shall be included in computing interest in connection with such payment.

        M. Certain Waivers. Borrower and each endorser hereby waive presentment,
notice of nonpayment or dishonor, protest, notice of protest and all other
notices in connection with the delivery, acceptance, performance, default or
enforcement of payment of this Note, and hereby waive all notice or right of
approval of any extensions, renewals, modifications or forbearances which may be
allowed.

        N. JURY TRIAL WAIVER. BORROWER HEREBY WAIVES, AND HOLDER BY ITS
ACCEPTANCE HEREOF THEREBY WAIVES, TRIAL BY JURY IN ANY LEGAL PROCEEDING
INVOLVING, DIRECTLY OR INDIRECTLY, ANY MATTER (WHETHER SOUNDING IN TORT,
CONTRACT OR OTHERWISE) IN ANY WAY ARISING OUT OF OR RELATED TO THIS NOTE OR THE
RELATIONSHIP EVIDENCED HEREBY. THIS PROVISION IS A MATERIAL INDUCEMENT FOR
HOLDER TO ACCEPT AND RELY UPON THIS NOTE.

        O. Severability. If any provision of this Note shall be prohibited or
invalid, under applicable law, it shall be ineffective only to such extent,
without invalidating the remainder of this Note.

                                     - 11 -
<PAGE>

        P. Maximum Interest Rate. If the effective interest rate on this Note
would otherwise violate any applicable usury law, then the interest rate shall
be reduced to the maximum permissible rate and any payment received by the
Holder in excess of the maximum permissible rate shall be treated as a
prepayment of the principal of this Note.

               [Remainder of this page intentionally left blank.]

                                     - 12 -
<PAGE>

        IN WITNESS WHEREOF, Borrower has caused this Note to be executed by its
duly authorized officer as of the date first written above.

                                        VIROLOGIC, INC.

                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:

        [Signature page to Series C Convertible Secured Promissory Note.]

                                     - 13 -

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