<PAGE>
                                                                    Exhibit 4.12

      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 200,000 Shares of
                                     Common Stock, par value $.001 per share

Date: November 19, 2002

                                 VIROLOGIC, INC.
                             STOCK PURCHASE WARRANT

      THIS CERTIFIES THAT, for value received, UBS WARBURG LLC, 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, Two
Hundred Thousand (200,000) 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.


                                       1
<PAGE>
      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 and the holder is not obligated
to return such certificate for the placement of a legend thereon, 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 Section 8 hereof 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.

      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


                                       2
<PAGE>
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 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


                                       3
<PAGE>
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]

            (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


                                       4
<PAGE>
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 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


                                       5
<PAGE>
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 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,


                                       6
<PAGE>
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 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


                                       7
<PAGE>
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. 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 set forth therein.

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


                                       8
<PAGE>
            (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 4.99%
of the outstanding shares of Common Stock. 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.

      8. Registration Rights. The Company agrees to include the Warrant Shares
in the initial registration statement to be filed by the Company in connection
with the issuance of its Series C Convertible Preferred Stock, subject to the
terms and obligations set forth in the Registration Rights Agreement dated
November 19, 2002 by and among the Company and the purchasers of the Company's
Series C Convertible Preferred Stock (the "RIGHTS AGREEMENT"),


                                       9
<PAGE>
including the right to assign such rights to certain assignees, as set forth
therein. The Holder hereby agrees to be bound by the obligations set forth in
the Rights Agreement to the same extent as if the Holder was an Investor
thereunder.

      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.

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


                                       11
<PAGE>
                  (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:_________________________


                                         UBS WARBURG LLC



                                         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:

                                          _____________________________________

                                          _____________________________________

                                          _____________________________________
<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.