<PAGE>   1
                                                                     EXHIBIT 4.6

     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 69,667 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 No. 1 (the "Holder") and upon the terms and subject to the
conditions hereinafter set forth, 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 69,667 shares (the "Shares") of either Preferred Stock
or Common Stock (the capital stock acquirable hereunder, the "Capital Stock).
In the event that the Company completes a sale of Preferred Stock (an
"Offering") on or before March 31, 1998, this Warrant shall be exercisable for
such Preferred Stock and the purchase price per Share shall be the price paid
by investors in the Offering. In the event that the Company completes an
Offering after March 31, 1998 and on or before June 30, 1998, this Warrant
shall be exercisable for Common  Stock and the purchase price per share shall
be the price paid by investors in the Offering, provided the price per share is
not less than $2.75 and not greater than $4.00. If an Offering does not occur
on or before June 30, 1998, this Warrant shall be exercisable for Common Stock
and the purchase price per Share shall be $4.00. 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.

     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.

     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

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<PAGE>   2
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 tie 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 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



                                       2

<PAGE>   3
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.

      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



                                       3
<PAGE>   4
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:

     (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



                                       4

<PAGE>   5
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 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.

     10.1 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.

     10.2 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 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.

     10.3 Right to Convert Warrant into Stock; Net Issuance.

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<PAGE>   6
          (a) 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.

          (b) 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. 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.


                                       6
<PAGE>   7
          (c)  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:

               (i)  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.

               (ii) 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, 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

                                       7
<PAGE>   8
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
changed, 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 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.

                                       8



<PAGE>   9

     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 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.

                                       9
<PAGE>   10

     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

                                        VIROLOGIC, INC.

                                        By: /s/ MARTIN H. GOLDSTEIN
                                            -----------------------------------

                                        Title: Pres & CEO
                                               --------------------------------

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


AGREED AND ACCEPTED:

MMC/GATX PARTNERSHIP NO. 1

By: Meier Mitchell & Company
    as General Partner

By:
   --------------------------------

Title:
      -----------------------------

                                       10
<PAGE>   11


     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

                                        VIROLOGIC, INC.

                                        By:
                                            -----------------------------------

                                        Title:
                                               --------------------------------

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


AGREED AND ACCEPTED:

MMC/GATX PARTNERSHIP NO. 1

By: Meier Mitchell & Company
    as General Partner

By: /s/ [Signature Illegible]
   --------------------------------

Title: Secretary
      -----------------------------

                                       10
<PAGE>   12
                                   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 or 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)





                                       11

<PAGE>   13
                                  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:

          o  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

          o  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)





                                       12

<PAGE>   14
                                   EXHIBIT B

                               CHARTER DOCUMENTS














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