<PAGE>
                                                                   EXHIBIT 4.13

NEITHER THIS WARRANT NOR THE SHARES OF STOCK ISSUABLE UPON EXERCISE HEREOF HAVE
BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED OR ANY STATE
SECURITIES LAWS. NO SALE, TRANSFER OR DISPOSITION OF THIS WARRANT OR SAID SHARES
MAY BE EFFECTED WITHOUT (i) EFFECTIVE REGISTRATION STATEMENTS RELATED THERETO,
(ii) AN OPINION OF COUNSEL OR OTHER EVIDENCE, REASONABLY SATISFACTORY TO THE
COMPANY, THAT SUCH REGISTRATIONS ARE NOT REQUIRED (iii) RECEIPT OF NO-ACTION
LETTERS FROM THE SECURITIES AND EXCHANGE COMMISSION TO THE EFFECT THAT
REGISTRATION UNDER THE ACT IS NOT REQUIRED OR (iv) OTHERWISE COMPLYING WITH THE
PROVISIONS OF SECTION 7 OF THIS WARRANT.


                                 VIROLOGIC, INC.

                        WARRANT TO PURCHASE 20,900 SHARES

                                OF CAPITAL STOCK

        THIS CERTIFIES THAT, pursuant to that certain Loan and Security
Agreement between VIROLOGIC, INC., a Delaware Corporation (the "COMPANY") and
MMC/GATX Partnership and upon the terms and subject to the conditions
hereinafter set forth, GATX FINANCIAL CORPORATION (the "HOLDER") may at any time
on or after the date of this Warrant and on or prior to expiration, subscribe
for and purchase from the Company up to 20,900 shares (the "SHARES") of Common
Stock (the capital stock acquirable hereunder, the "CAPITAL STOCK") and the
purchase price per Share shall be $8.00. As used herein, (a) the term "DATE OF
GRANT" shall mean January 30, 1998, and (b) the term "OTHER WARRANTS" shall mean
any other warrants issued by the Company in connection with the transaction with
respect to which this Warrant was issued, and any warrant issued upon transfer
or partial exercise of this Warrant. The term "WARRANT" as used herein shall be
deemed to include Other Warrants unless the context clearly requires otherwise.
This Warrant replaces the original warrant dated January 30, 1998.

      1.    TERM. The purchase right represented by this Warrant is exercisable,
in whole or in part, at any time and from time to time from the Date of Grant
through the later of (i) ten (10) years after the Date of Grant or (ii) five (5)
years after the closing of the Company's initial public offering of its Common
Stock effected pursuant to a Registration Statement on Form S-1 (or its
successor) filed under the Securities Act of 1933, as amended (the "ACT"). Upon
request of the Company, the holder of this Warrant agrees to exercise the
purchase right under this Warrant (including without limitation by way of net
issuance as provided in paragraph 10.3) upon the sale of all of the assets or
stock of the Company, or the merger of the Company if the net proceeds per share
to the holder of this Warrant upon such exercise would equal at least the
product of the Warrant Price multiplied by 2.0. The Company agrees to provide
the holder of this Warrant not less than thirty (30) days' prior written notice
of the Company's request that the holder exercise its purchase right hereunder
in accordance with the provisions of Section 14 hereof.


                                       1.
<PAGE>
      2.    METHOD OF EXERCISE; PAYMENT; ISSUANCE OF NEW WARRANT. Subject to
Section 1 hereof, the purchase right represented by this Warrant may be
exercised by the holder hereof, in whole or in part and from time to time, at
the election of the holder hereof, by (a) the surrender of this Warrant (with
the notice of exercise substantially in the form attached hereto as Exhibit A
duly completed and executed) at the principal office of the Company and by the
payment to the Company, by cash, certified or bank check, or by wire transfer to
an account designated by the Company (a "WIRE TRANSFER") of an amount equal to
the then applicable Warrant Price multiplied by the number of Shares then being
purchased, or (b) if in connection with a registered public offering of the
Company's securities, the surrender of this Warrant (with the notice of exercise
form attached hereto as Exhibit A-1 duly completed and executed) at the
principal office of the Company together with notice of arrangements reasonably
satisfactory to the Company for payment to the Company either by cash, certified
or bank check or by Wire Transfer from the proceeds of the sale of shares to be
sold by the holder in such public offering of an amount equal to the then
applicable Warrant Price per share multiplied by the number of Shares then being
purchased or (c) exercise of the right provided for in Section 10.3 hereof. The
person or persons in whose name(s) any certificate(s) representing the Shares
shall be issuable upon exercise of the Warrant shall be deemed to have become
the holder(s) of record of, and shall be treated for all purposes as the record
holder(s) of, the shares represented thereby (and such shares shall be deemed to
have been issued) immediately prior to the close of business on the date or
dates upon which this Warrant is exercised. In the event of any exercise of the
rights represented by this Warrant, certificates for the shares of stock so
purchased shall be delivered to the holder hereof as soon as possible and in any
event within thirty (30) days after such exercise and, unless this Warrant has
been fully exercised or expired, a new Warrant representing the portion of the
Shares, if any, with respect to which this Warrant shall not then have been
exercised shall also be issued to the holder hereof as soon as possible and in
any event within such thirty-day period.

      3.    STOCK FULLY PAID; RESERVATION OF SHARES. All Shares that may be
issued upon the exercise of this Warrant will, upon issuance pursuant to the
terms and conditions herein, be fully paid and nonassessable, and free from all
taxes, liens and charges with respect to the issue thereof. During the period
within which this Warrant may be exercised, the Company will at all times have
authorized and reserved, for the purpose of this issuance upon exercise of this
Warrant, a sufficient number of shares of its Capital Stock to provide for the
exercise of this Warrant, and, if this Warrant is for the acquisition of
Preferred Stock, a sufficient number of shares of its Common Stock to provide
for the conversion of Preferred Stock into Common Stock.

      4.    ADJUSTMENT OF WARRANT PRICE AND NUMBER OF SHARES. The number and
kind of securities purchasable upon the exercise of this Warrant and the Warrant
Price shall be subject to adjustment from time to time upon the occurrence of
certain events, as follows:

            (a) RECLASSIFICATION OR MERGER. In case of any reclassification or
change of securities of the class issuable upon exercise of this Warrant (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), or in case
of any merger of the Company with or into another corporation (other than a
merger with another corporation in which the Company is the acquiring and the
surviving corporation and which does not result in any reclassification or
change of outstanding securities

                                       2.
<PAGE>
issuable upon exercise of this Warrant), or in case any sale of all or
substantially all of the assets of the Company, the Company, or such successor
or purchasing corporation, as the case may be, shall duly execute and deliver to
the holder of this Warrant a new Warrant (in form and substance satisfactory to
the holder of the Warrant), so that (1) the holder of this Warrant shall have
the right to receive at a total purchase price not to exceed that payable upon
the exercise of the unexercised portion of this Warrant and (2) upon such
exercise to receive, in lieu of the shares of Capital Stock, theretofore
issuable upon exercise of this Warrant, the kind and amount of shares of stock,
other securities, money and property receivable upon such reclassification,
change or merger by a holder of the number of shares of Capital Stock then
purchasable under this Warrant. Such new Warrant shall provide for adjustments
that shall be as nearly equivalent as may be practicable to the adjustments
provided for in this Section 4. The provisions of this subparagraph (a) shall
similarly apply to successive reclassifications, changes, mergers and transfers.

            (b) SUBDIVISION OR COMBINATION OF SHARES. If the Company at any time
while this Warrant remains outstanding and unexpired shall subdivide or combine
its Capital Stock, the Warrant Price shall be proportionately decreased in the
case of a subdivision or increased in the case of a combination, effective at
the close of business on the date the subdivision or combination becomes
effective.

            (c) STOCK DIVIDENDS AND OTHER DISTRIBUTIONS. If the Company at any
time while this Warrant is outstanding and unexpired shall (i) pay a dividend
with respect to Capital Stock payable in Capital Stock, or (ii) make any other
distribution of Capital Stock (except any distribution specifically provided for
in Sections 4(a) and 4(b)), then the Warrant Price shall be adjusted, from and
after the date of determination of shareholders entitled to receive such
dividend or distribution, to that price determined by multiplying the Warrant
Price in effect immediately prior to such date of determination by a fraction
(i) the numerator of which shall be the total number of shares of Capital Stock
outstanding immediately prior to such dividend or distribution, and (ii) the
denominator of which shall be the total number of shares of Capital Stock
outstanding immediately after such dividend or distribution.

            (d) ADJUSTMENT OF NUMBER OF SHARES. Upon each adjustment in the
Warrant Price, the number of Shares purchasable hereunder shall be adjusted, to
the nearest whole share, to the product obtained by multiplying the number of
Shares purchasable immediately prior to such adjustment in the Warrant Price by
a fraction, the numerator of which shall be the Warrant Price immediately prior
to such adjustment and the denominator of which shall be the Warrant Price
immediately thereafter.

      5.    NOTICE OF ADJUSTMENTS. Whenever the Warrant Price or the number of
Shares purchasable hereunder shall be adjusted pursuant to Section 4 hereof, the
Company shall make a certificate signed by its chief financial officer setting
forth, in reasonable detail, the event requiring the adjustment, the amount of
the adjustment, the method by which such adjustment was calculated, and the
Warrant Price and the number of Shares purchasable hereunder after giving effect
to such adjustment, and shall cause copies of such certificate to be mailed
(without regard to Section 13 hereof, by first class mail, postage prepaid) to
the holder of this Warrant at such holder's last known address.


                                       3.
<PAGE>
      6.    FRACTIONAL SHARES. No fractional Shares of Capital Stock will be
issued in connection with any exercise hereunder, but in lieu of such fractional
shares the Company shall make a cash payment therefor equal to the product of
such fraction multiplied by the fair market value of one share of Capital Stock
on the date of exercise, as reasonably determined in good faith by the Company's
Board of Directors.

      7.    COMPLIANCE WITH SECURITIES ACT; DISPOSITION OF WARRANT OR SHARES OF
COMMON STOCK.

            (a) COMPLIANCE WITH SECURITIES ACT. The holder of this Warrant, by
acceptance hereof, agrees that this Warrant and the Shares to be issued upon
exercise hereof are being acquired for investment and that such holder will not
offer, sell or otherwise dispose of this Warrant or any Shares to be issued upon
exercise hereof except under circumstances which will not result in a violation
of the Securities Act of 1933, as amended (the "ACT") or any applicable state
securities laws. Upon exercise of this Warrant, unless the Shares being acquired
are registered under the Act and any applicable state securities laws or an
exemption from such registration is available, the holder hereof shall confirm
in writing, in a form reasonably satisfactory to the Company, the Shares so
purchased are being acquired for investment and not with a view toward
distribution or resale in violation of the Act and shall confirm such other
matters related thereto as may be reasonably requested by the Company. This
Warrant and all Shares issued upon exercise of this Warrant (unless registered
under the Act and any applicable state securities laws) shall be stamped or
imprinted with a legend in substantially the following form:

                   "THE SECURITIES EVIDENCED HEREBY HAVE NOT BEEN REGISTERED
                   UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR ANY STATE
                   SECURITIES LAWS. NO SALE OR DISPOSITION MAY BE AFFECTED
                   WITHOUT (i) AN EFFECTIVE REGISTRATION STATEMENT RELATED
                   THERETO, (ii) AN OPINION OF COUNSEL FOR THE HOLDER,
                   REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH
                   REGISTRATION IS NOT REQUIRED, (iii) RECEIPT OF A NO-ACTION
                   LETTER FROM THE SECURITIES AND EXCHANGE COMMISSION TO THE
                   EFFECT THAT REGISTRATION UNDER THE ACT IS NOT REQUIRED, OR
                   (iv) OTHERWISE COMPLYING WITH THE PROVISIONS OF SECTION 7 OF
                   THE WARRANT UNDER WHICH THESE SECURITIES WERE ISSUED,
                   DIRECTLY OR INDIRECTLY."

                   Said legend shall be removed by the Company, upon the request
of a holder, at such time as the restrictions on the transfer of the applicable
security shall have terminated. In addition, in connection with the issuance of
this Warrant, the holder specifically represents to the Company by acceptance of
this Warrant as follows:


                                       4.
<PAGE>
                  (1) The holder is aware of the Company's business affairs and
financial condition, and has acquired information about the Company sufficient
to reach an informed and knowledgeable decision to acquire this Warrant. The
holder is acquiring this Warrant for its own account for investment purposes
only and not with a view to, or for the resale in connection with, any
"distribution" thereof in violation of the Act.

                  (2) The holder understands that this Warrant has not been
registered under the Act and that the shares of stock acquired pursuant to the
exercise of this Warrant will be "restricted securities" within the meaning of
Rule 144 under the Act.

                  (3) The holder further understands that this Warrant must be
held indefinitely unless subsequently registered under the Act and qualified
under any applicable state securities laws, or unless exemptions from
registration and qualification are otherwise available. The holder is aware of
the provisions of Rule 144, promulgated under the Act.

            (b) DISPOSITION OF WARRANT OR SHARES. With respect to any offer,
sale or other disposition of this Warrant or any Shares acquired pursuant to the
exercise of this Warrant prior to registration thereof, the holder hereof and
each subsequent holder agrees to give written notice to the Company prior
thereto, describing briefly the manner thereof, together with a written opinion
of such holder's counsel, if reasonably requested by the Company, to the effect
that such offer, sale or other disposition may be effected without registration
or qualification (under the Act as then in effect or any federal or state
securities law then in effect) of this Warrant or the Shares and indicating
whether or not under the Act certificates for this Warrant or the Shares to be
sold or otherwise disposed of require any restrictive legend as to applicable
restrictions on transferability in order to ensure compliance with the Act.
Promptly upon receiving such written notice and reasonably satisfactory opinion,
if so requested, the Company, as promptly as practicable but no later than
fifteen (15) days after receipt of the written notice, shall notify such holder
that such holder may sell or otherwise dispose of this Warrant or such Shares,
all in accordance with the terms of the notice delivered to the Company. If a
determination has been made pursuant to this Section 7(b) that the opinion of
counsel for the holder or other evidence is not reasonably satisfactory to the
Company, the Company shall so notify the holder promptly with details thereof
after such determination has been made. Notwithstanding the foregoing, this
Warrant or such Shares may be offered, sold or otherwise disposed of in
accordance with Rule 144 or 144A under the Act, provided that the Company shall
have been furnished with such information as the Company may reasonably request
to provide a reasonable assurance that the provisions of Rule 144 or 144A have
been satisfied. Each certificate representing this Warrant or the Shares thus
transferred (except a transfer pursuant to Rule 144 or 144A) shall bear a legend
as to the applicable restrictions on transferability in order to ensure
compliance with the Act, unless, in the aforesaid opinion of counsel for the
holder, such legend is not required in order to ensure compliance with the Act.
The Company may issue stop transfer instructions to its transfer agent in
connection with such restrictions. This Warrant may not be transferred or
assigned without the prior consent of the Company.

            (c) APPLICABILITY OF RESTRICTIONS. The restrictions set forth in
Section 7(c) hereto shall not restrict up to two (2) transfers of this Warrant
or any part hereof by the initial holder (i) to any partnership affiliated with
the initial holder, (ii) to any partner of any such

                                       5.
<PAGE>
partnership, (iii) to any affiliate of any such parties, or (iv) to any
investment bank, for purposes of liquidating the investment represented by this
Warrant, provided such transfer may be made in compliance with applicable
federal and state securities laws; provided, however, in any such transfer, if
applicable, the transferee shall on the Company's request agree in writing to be
bound by the terms of this Warrant as if an original signatory hereto.

      8.    RIGHTS AS SHAREHOLDERS; INFORMATION. No holder of this Warrant, as
such, shall be entitled to vote or receive dividends or be deemed the holder of
Shares, nor shall anything contained herein be construed to confer upon the
holder of this Warrant, as such, any of the rights of a shareholder of the
Company or any right to vote for the election of directors or upon any matter
submitted to shareholders at any meeting thereof, or to give or withhold consent
to any corporate action (whether upon any recapitalization, issuance of stock,
reclassification of stock, change of par value or change of stock to no par
value, consolidation, merger, conveyance or otherwise), or to receive notice of
meetings, or to receive dividends or subscription rights or otherwise until this
Warrant shall have been exercised and the Shares purchasable upon the exercise
hereof shall have become deliverable, as provided herein. Notwithstanding the
foregoing, the Company will transmit to the holder of this Warrant such
information, documents and reports as are generally distributed to the holders
of any class or series of the securities of the Company concurrently with the
distribution thereof to the shareholders.

      9.    REGISTRATION RIGHTS. The Company grants registration rights to the
holder of this Warrant for any Common Stock of the Company obtained upon
exercise hereof, comparable to the registration rights granted to the investors
in that certain ViroLogic, Inc. Investors' Rights Agreement dated as of May 23,
1996, (the "REGISTRATION RIGHTS AGREEMENT"), with the following exceptions:

                  (1) The holder will have no demand registration rights.

                  (2) The holder will be subject to the same provisions
regarding indemnification as contained in the Registration Rights Agreement.

                  (3) The registration rights are freely assignable by the
holder of this Warrant.

      10.   ADDITIONAL RIGHTS.

            (a) SECONDARY SALES. The Company agrees that it will not interfere
with the holder of this Warrant in obtaining liquidity if opportunities to make
secondary sales of the Company's securities become available. To this end, the
Company will promptly provide the holder of this Warrant with notice of any
offer (of which it has knowledge) to acquire from the Company's security holders
more than five percent (5%) of the total voting power of the Company and will
not interfere with any attempt by the holder in arranging the sale of this
Warrant to the person or persons making such offer.

            (b) MERGERS. The Company shall provide the holder of this Warrant
with at least thirty (30) days' notice of the terms and conditions of any of the
following potential transactions: (i) the sale, lease, exchange, conveyance or
other disposition of all or substantially all of the Company's property or
business, or (ii) its merger into or consolidation with any other

                                       6.
<PAGE>
corporation (other than a wholly-owned subsidiary of the Company), or any
transaction (including a merger or other reorganization) or series of related
transactions, in which more than 50% of the voting power of the Company is
disposed of. The Company will cooperate with the holder in arranging the sale of
this Warrant in connection with any such transaction.

            (c) RIGHT TO CONVERT WARRANT INTO STOCK; NET ISSUANCE.

                  (1) RIGHT TO CONVERT. In addition to and without limiting the
rights of the holder under the terms of this Warrant, the holder shall have the
right to convert this Warrant or any portion thereof (the "CONVERSION RIGHT")
into shares of Capital Stock as provided in this Section 10.3 at any time or
from time to time during the term of this Warrant. Upon exercise of the
Conversion Right with respect to a particular number of shares subject to this
Warrant (the "CONVERTED WARRANT SHARES"), the Company shall deliver to the
holder (without payment by the holder of any exercise price or any cash or other
consideration) (X) that number of shares of fully paid and nonassessable Capital
Stock equal to the quotient obtained by dividing the value of this Warrant (or
the specified portion hereof) on the Conversion Date (as defined in subsection
(b) hereof), which value shall be determined by subtracting (A) the aggregate
Warrant Price of the Converted Warrant Shares immediately prior to the exercise
of the Conversion Right from (B) the aggregate fair market value of the
Converted Warrant Shares issuable upon exercise of this Warrant (or the
specified portion hereof) on the Conversion Date (as herein defined) by (Y) the
fair market value of one share of Capital Stock on the Conversion Date (as
herein defined).

        Expressed as a formula, such conversion shall be computed as follows:

        X = B - A
            _____
              Y

      Where:    X = the number of shares of Capital Stock that may be issued to
                    holder

                Y = the fair market value (FMV) of one share of Capital Stock

                A = the aggregate Warrant Price (i.e., Converted Warrant Shares
                    x Warrant Price)

                B = the aggregate FMV (i.e., FMV x Converted Warrant Shares)

      No fractional shares shall be issuable upon exercise of the Conversion
Right, and, if the number of shares to be issued determined in accordance with
the foregoing formula is other than a whole number, the Company shall pay to the
holder an amount in cash equal to the fair market value of the resulting
fractional share on the Conversion Date (as hereinafter defined). For purposes
of Section 9 of this Warrant, shares issued pursuant to the Conversion Right
shall be treated as if they were issued upon the exercise of this Warrant.

            (d) METHOD OF EXERCISE. The Conversion Right may be exercised by the
holder by the surrender of this Warrant at the principal office of the Company
together with a written statement specifying that the holder thereby intends to
exercise the Conversion Right and indicating the number of shares subject to
this Warrant which are being surrendered (referred to in Section 10.3(a) hereof
as the Converted Warrant Shares) in exercise of the Conversion Right.

                                       7.
<PAGE>
Such conversion shall be effective upon receipt by the Company of this Warrant
together with the aforesaid written statement, or on such later date as is
specified therein (the "CONVERSION DATE"), and, at the election of the holder
hereof, may be made contingent upon the closing of the sale of the Company's
Common Stock to the public in a public offering pursuant to a Registration
Statement under the Act (a "PUBLIC OFFERING"). Certificates for the shares
issuable upon exercise of the Conversion Right and, if applicable, a new warrant
evidencing the balance of the shares remaining subject to this Warrant, shall be
issued as of the Conversion Date and shall be delivered to the holder within
thirty (30) days following the Conversion Date.

            (e) DETERMINATION OF FAIR MARKET VALUE. For purposes of this Section
10.3, "fair market value" of a share of Capital Stock as of a particular date
(the "DETERMINATION DATE") shall mean:

                  (1) If the Conversion Right is exercised in connection with
and contingent upon a Public Offering, and if the Company's Registration
Statement relating to such Public Offering ("REGISTRATION STATEMENT") has been
declared effective by the SEC, then the initial "Price to Public" specified in
the final prospectus with respect to such offering.

                  (2) If the Conversion Right is not exercised in connection
with and contingent upon a Public Offering, then as follows:

                        (A) If traded on a securities exchange or the Nasdaq
National Market, the fair market value of the Capital Stock shall be deemed to
be the average of the closing prices of the Common Stock on such exchange or
market over the 20-day period ending five business days ending immediately prior
to the applicable Determination Date;

                        (B) If traded over-the-counter, the fair market value of
the Capital Stock shall be deemed to be the average of the closing bid prices of
the Common Stock over the 20-day period ending five days ending immediately
prior to the applicable Determination Date; and

                        (C) If there is no public market for the Common Stock,
then fair market value shall be determined in good faith by the board of
directors of the Company.

      11.   "MARKET STAND-OFF" AGREEMENT. Each holder by acceptance of this
Warrant agrees that, during a period not to exceed 180 days, following the
effective date of a registration of the Company filed under the Act, it shall
not, to the extent requested by the Company or the underwriters managing such
underwritten public offering of the Company's securities, sell or otherwise
transfer or dispose of any securities of the Company held by it at any time
during such period except securities included in such registration; provided,
however, that all officers and directors of the Company and all stockholders of
the Company enter into similar agreements.

      12.   REPRESENTATIONS AND WARRANTIES. The Company represents and warrants
to the holder of this Warrant as follows:

            (a) This Warrant has been duly authorized and executed by the
Company and is a valid and binding obligation of the Company enforceable in
accordance with its terms,

                                       8.
<PAGE>
subject to laws of general application relating to bankruptcy, insolvency and
the relief of debtors and the rules of law or principles at equity governing
specific performance, injunctive relief and other equitable remedies;

            (b) The Shares have been duly authorized and reserved for issuance
by the Company and, when issued in accordance with the terms hereof will be
validly issued, fully paid and non-assessable;

            (c) The rights, preferences, privileges and restrictions granted to
or imposed upon the Shares and the holders thereof are as set forth in the
Company's Restated Certificate of Incorporation, as amended to the Date of the
Grant, a true and complete copy of which has been delivered to the original
holder of this Warrant and is attached hereto as Exhibit B (the "CHARTER");

            (d) The execution and delivery of this Warrant are not, and the
issuance of the Shares upon exercise of this Warrant in accordance with the
terms hereof will not be, inconsistent with the Company's Charter or by-laws, do
not and will not contravene any law, governmental rule or regulation, judgment
or order applicable to the Company, and do not and will not conflict with or
contravene any provision of, or constitute a default under, any indenture,
mortgage, contract or other instrument of which the Company is a party or by
which it is bound or require the consent or approval of, the giving of notice
to, the registration or filing with or the taking of any action in respect of or
by, any Federal, state or local government authority or agency or other person,
except for the filing of notices pursuant to federal and state securities laws,
which filings will be effected by the time required thereby; and

            (e) There are no actions, suits, audits, investigations or
proceedings pending or, to the knowledge of the Company, threatened against the
Company in any court or before any governmental commission, board or authority
which, if adversely determined, will have a material adverse effect on the
ability of the Company to perform its obligations under this Warrant.

      13.   MODIFICATION AND WAIVER. This Warrant and any provision hereof may
be hanged, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought.

      14.   NOTICES. Any notice, request, communication or other document
required or permitted to be given or delivered to the holder hereof or the
Company shall be delivered, or shall be sent by certified or registered mail,
postage prepaid, to each such holder at its address as shown on the books of the
Company or to the Company at the address indicated therefor on the signature
page of this Warrant.

      15.   BINDING EFFECT ON SUCCESSORS. This Warrant shall be binding upon any
corporation succeeding the Company by merger, consolidation or acquisition of
all or substantially all of the Company's assets, and all of the obligations of
the Company relating to the Shares issuable upon the exercise or conversion of
this Warrant shall survive the exercise, conversion and termination of this
Warrant and all of the covenants and agreements of the Company shall inure to
the benefit of the successors and assigns of the holder hereof. The

                                       9.
<PAGE>
Company will, at the time of the exercise or conversion of this Warrant, in
whole or in part, upon request of the holder hereof but at the Company's
expense, acknowledge in writing its continuing obligation to the holder hereof
in respect of any rights (including, without limitation, any right to
registration of the Shares) to which the holder hereof shall continue to be
entitled after such exercise or conversion in accordance with this Warrant;
provided, that the failure of the holder hereof to make any such request shall
not affect the continuing obligation of the Company to the holder in respect of
such rights.

      16.   LOST WARRANTS OR STOCK CERTIFICATES. The Company covenants to the
holder hereof that, upon receipt of evidence reasonably satisfactory to the
Company of the loss, theft, destruction or mutilation of this Warrant or any
stock certificate and, in the case of any such loss, theft or destruction, upon
receipt of an indemnity reasonably satisfactory to the Company, or in the case
of any such mutilation upon surrender and cancellation of such Warrant or stock
certificate, the Company will make and deliver a new Warrant or stock
certificate, or like tenor, in lieu of the lost, stolen, destroyed or mutilated
Warrant or stock certificate.

      17.   DESCRIPTIVE HEADINGS. The descriptive headings of the several
paragraphs of this Warrant are inserted for convenience only and do not
constitute a part of this Warrant. The language in this Warrant shall be
construed as to its fair meaning without regard to which party drafted this
Warrant.

      18.   GOVERNING LAW. This Warrant shall be construed and enforced in
accordance with, and the rights of the parties shall be governed by, the laws of
the State of California.

      19. SURVIVAL OF REPRESENTATIONS, WARRANTIES AND AGREEMENTS. All
representations and warranties of the Company and the holder hereof contained
herein shall survive the Date of Grant, the exercise or conversion of this
Warrant (or any part hereof) or the termination or expiration of rights
hereunder. All agreements of the Company and the holder hereof contained herein
shall survive indefinitely until, by their respective terms, they are no longer
operative.

      20.   REMEDIES. In case any one or more of the covenants and agreements
contained in this Warrant shall have been breached, the holders hereof (in the
case of a breach by the Company), or the Company (in the case of a breach by a
holder), may proceed to protect and enforce their or its rights either by suit
in equity and/or by action at law, including, but not limited to, an action for
damages as a result of any such breach and/or action for specific performance of
any such covenant or agreement contained in this Warrant.

      21.   NO IMPAIRMENT OF RIGHTS. The Company will not, by amendment of its
Charter or through any other means, avoid or seek to avoid the observance or
performance of any of the terms of this Warrant, but will at all times in good
faith assist in the carrying out of all such terms and in the taking of all such
actions as may be necessary or appropriate in order to protect the rights of the
holder of this Warrant against impairment.

      22.   SEVERABILITY. The invalidity or unenforceability of any provision of
this Warrant in any jurisdiction shall not affect the validity or enforceability
of such provision in any

                                      10.
<PAGE>
other jurisdiction, or affect any other provision of this Warrant, which shall
remain in full force and effect.

      23.   RECOVERY OF LITIGATION COSTS. If any legal action or other
proceeding is brought for the enforcement of this Warrant, or because of an
alleged dispute, breach, default, or misrepresentation in connection with any of
the provisions of this Warrant, the successful or prevailing party or parties
shall be entitled to recover reasonable attorneys' fees and other costs incurred
in that action or proceeding, in addition to any other relief to which it or
they may be entitled.

      24.   ENTIRE AGREEMENT; MODIFICATION. This Warrant constitutes the entire
agreement between the parties pertaining to the subject matter contained in it
and supersedes all prior and contemporaneous agreements, representations, and
undertakings of the parties, whether oral or written, with respect to such
subject matter.

Date of Grant:  January 30, 1998

Date of Reissuance:  April 23, 2002

                                   VIROLOGIC, INC.



                                   By: /s/ William D. Young
                                       ---------------------------------------
                                       William D. Young
                                       President and Chief Executive Officer

                                   Address:
                                   270 E. Grand Avenue
                                   South San Francisco, CA 94080


AGREED AND ACCEPTED:

GATX FINANCIAL CORPORATION


By:    /s/ Patricia W. Leicher
       ------------------------------------------

Title:  President
        ----------------------------------------

Address:
Four Embarcadero Center, Suite 2200
San Francisco, CA 94111


                                      11.
<PAGE>
                                    EXHIBIT A

                               NOTICE OF EXERCISE


To:  VIROLOGIC, INC. (the "Company")

      1.    The undersigned hereby:

            -     elects to purchase _____________ shares of Common Stock of the
                  Company pursuant to the terms of the attached Warrant, and
                  tenders herewith payment of the purchase price of such shares
                  in full, or

            -     elects to exercise its net issuance rights pursuant to Section
                  10.3 of the attached Warrant with respect to _______ shares of
                  Common Stock .

      2.    Please issue a certificate of certificates representing said
shares in the name of the undersigned or in such other name or names as are
specified below:

                                   ________________________________________
                                     (Name)

                                   ________________________________________

                                   ________________________________________
                                    (Address)

      3.    The undersigned represents that the aforesaid shares are being
acquired for the account of the undersigned for investment and not with a view
to, or for resale in connection with, the distribution thereof and that the
undersigned has no present intention of distributing or reselling such shares,
all except as in compliance with applicable securities laws.


                                   ________________________________________
                                                  (Signature)

____________________
         (Date)



                                      12.
<PAGE>
                                   EXHIBIT A-1

                               NOTICE OF EXERCISE


To:  VIROLOGIC, INC. (the "Company")

      1.    Contingent upon and effective immediately prior to the closing (the
"Closing") of the Company's public offering contemplated by the Registration
Statement on Form S____, filed _________________, 19__, the undersigned hereby:

      -     elects to purchase _________ shares of Common Stock of the Company
            (or such lesser number of shares as may be sold on behalf of the
            undersigned at the Closing) pursuant to the terms of the attached
            Warrant, or

      -     elects to exercise its net issuance rights pursuant to Section 10.3
            of the attached Warrant with respect to _________ shares of Common
            Stock.

      2.    Please deliver to the custodian for the selling shareholders a stock
certificate representing such __________ shares.

      3.    The undersigned has instructed the custodian for the selling
shareholders to deliver to the Company $____________ or, if less, the net
proceeds due the undersigned from the sale of shares in the aforesaid public
offering. If such net proceeds are less than the purchase price for such shares,
the undersigned agrees to deliver the difference to the Company prior to the
Closing.

                                     ___________________________________________
                                                     (Signature)

 __________________
         (Date)


                                      13.
<PAGE>
                                    EXHIBIT B

                                CHARTER DOCUMENTS


                                      14.