Document:

EX-4.3

 Exhibit 4.3 

EXAGEN DIAGNOSTICS, INC. 

FIFTH AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

THIS FIFTH AMENDED AND RESTATED STOCKHOLDERS’ AGREEMENT (this “Agreement”) is made as of January 19, 2016, by and
among Exagen Diagnostics, Inc., a Delaware corporation (the “Corporation”), and the persons listed on Schedule A hereto (the “Investors”). 

RECITALS 
 WHEREAS, the
Investors are party to that certain Fourth Amended and Restated Stockholders Agreement of the Corporation dated October 4, 2013 (as amended, the “Prior Agreement”); 

WHEREAS, certain of the Investors and the Corporation have entered into that certain Series E Preferred Stock Purchase and Exchange Agreement
dated the date hereof (the “Series E Purchase Agreement”), pursuant to which such Investors agreed to purchase from the Corporation, and the Corporation agreed to sell to such Investors, shares of the Corporation’s Series E
Convertible Preferred Stock, par value $0.001 per share (the “Series E Preferred”); 
 WHEREAS, the Prior Agreement
permits the Corporation, the record or beneficial Investors owning at least two thirds (2/3) of the issued and outstanding shares of Series D Convertible Preferred Stock, par value $0.001 per share (the “Series D Preferred
Stock”) and the record or beneficial Investors owning at least two-thirds (2/3) of the issued and outstanding shares of Series C Convertible Preferred Stock, par value $0.001 per share (the “Series C Preferred”), to
amend the Prior Agreement; and 
 WHEREAS, as a condition to the consummation of the transactions contemplated by the Series E Purchase
Agreement, the undersigned Investors, who are the record or beneficial Investors owning at least two-thirds (2/3) of the shares of Series D Preferred outstanding as of the date hereof and the record or beneficial Investors owning at least two
thirds (2/3) of the shares of Series C Preferred outstanding as of the date hereof, and the Corporation desire to amend and restate the Prior Agreement in its entirety as set forth herein, to provide for the future voting of the Investors’
shares of Capital Stock and to set forth their respective rights and obligations relating to certain transfers of such shares, in each case as set forth herein with the intention that this Agreement shall become effective concurrently with the
Initial Closing (as defined in the Series E Purchase Agreement). 
 AGREEMENT 

NOW THEREFORE, in consideration of the foregoing and of the mutual promises and covenants contained herein and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereby agree as follows: 

 SECTION 1. Definitions. As used in this Agreement, the following terms shall have the
following respective meanings: 
 “Additional Senior Preferred Director” shall have the meaning set forth in
Section 4.1(a)(i)(B). 
 “Adversely Affected Investor” shall have the meaning set forth in Section 6.4. 

“Affiliate” shall mean, with respect to any Investor, any person or entity controlling, controlled by or under common
control with such Investor, and (i) with respect to vSpring SBIC, L.P., shall include any affiliate of vSpring Capital, LLC, (ii) with respect to Wasatch Venture Fund III, L.L.C., shall include any venture capital fund managed by the
general partners of Wasatch Venture Fund III, L.L.C., (iii) with respect to NMSIC Co-Investment Fund, L.P., shall include any affiliate of Sun Mountain Capital Partners, LLC, (iv) with respect to PCM/Exagen, L.P., shall include Hunt
Holdings L.P., and (v) with respect to Tullis-Dickerson Capital Focus III, L.P., shall include any pooled investment vehicle managed by Tullis-Dickerson and Co., Inc., or any of its affiliates. 

“Agreement” shall mean this Fifth Amended and Restated Stockholders’ Agreement, as the same may be amended from time to
time. 
 “Approved Action” shall have the meaning set forth in Section 6.1. 

“Board” shall mean the Board of Directors of the Corporation. 

“Capital Stock” shall mean the Common Stock and Preferred Stock. 

“Certificate” shall mean the Sixteenth Amended and Restated Certificate of Incorporation of the Corporation, as the same may
be amended from time to time. 
 “Common Director” shall have the meaning set forth in Section 4.1(a)(ii). 

“Common Stock” shall mean the Corporation’s common stock, $0.001 par value per share. 

“Co-Sale Investor” shall have the meaning set forth in Section 3.1. 

“Corporation” shall mean Exagen Diagnostics, Inc., a Delaware corporation, its successors and assigns. 

“Deemed Liquidation Event” shall have the meaning set forth in Section 5.1. 

“Electing Investor” shall have the meaning set forth in Section 2.2. 

“Equity Securities” shall mean, with respect to any Investor, any of the following that is now owned or that is hereinafter
acquired: (i) any Common Stock, Preferred Stock or other class or series of capital stock of the Corporation; (ii) any security convertible, with or without consideration, into any Common Stock, Preferred Stock or other class or series of
capital 

  
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stock of the Corporation; (iii) any security carrying any warrant or right to subscribe for or purchase shares of any class or series of capital stock of the Corporation; and (iv) any
warrant, note, right, option or other derivative security which provides the right to subscribe for or purchase any stock or securities of the types listed in any of clauses (i), (ii), or (iii) above. 

“Excluded Securities” shall mean Common Stock issued or issuable (a) pursuant to the Series E Purchase Agreement,
(b) upon conversion of the Preferred Stock, (c) to officers, directors or employees of, or consultants to, the Corporation pursuant to any stock option, incentive, bonus or compensation program approved by the Board or such other
arrangements, contracts, or plans as are recommended by management and approved by the Board, (d) by way of dividend or other distribution on shares of Preferred Stock or by way of distribution on shares of Common Stock or Preferred Stock
pursuant to stock splits, recapitalizations and similar transactions, (e) upon exercise of warrants to purchase shares of Preferred Stock or Common Stock, (f) in a transaction with respect to which the holders of at least 52% of the then
outstanding shares of Senior Preferred, voting together as a single class, have waived their rights under Section 2 hereof, (g) in connection with a business acquisition of or by the Corporation, (h) pursuant to equipment lease
financings or bank credit arrangements approved by the Board, (i) for charitable purposes approved by the Board, or (j) as part of a corporate partnering transaction approved by the Board. 

“Financing Securities” shall have the meaning set forth in Section 2.1. 

“Independent Directors” shall have the meaning set forth in Section 4.1(a)(iii). 

“Major Investor” and “Major Investors” shall have the meaning set forth in Section 2.1. 

“Non-electing Investor” shall have the meaning set forth in Section 2.2. 

“Notice of Acceptance” shall have the meaning set forth in Section 2.2. 

“Offer” shall have the meaning set forth in Section 2.1. 

“Option” shall mean any security convertible into, or exchangeable or exercisable for, shares of Common Stock. 

“PCM/Hunt” shall have the meaning set forth in Section 4.1(a)(i)(C). 

“PCM/Hunt Director” shall have the meaning set forth in Section 4.1(a)(i)(C). 

“Preferred Stock” shall mean the Series A-3 Preferred, the Series B-3 Preferred, the Series C Preferred, the Series D
Preferred and the Series E Preferred. 
 “Proposed Sale” shall have the meaning set forth in Section 5.3. 

“Proposed Transfer” shall mean any assignment, sale, offer to sell, pledge, mortgage, hypothecation, encumbrance,
disposition of or any other like transfer or encumbering of any Equity Securities proposed by any Investor. 

  
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 “Proposed Transfer Notice” shall mean written notice from an Investor setting
forth the terms and conditions of a Proposed Transfer. 
 “pro rata share” shall have the meaning set forth in
Section 2.1. 
 “Remaining Securities” shall have the meaning set forth in Section 2.3. 

“Right of Co-Sale” shall mean the right, but not an obligation, of an Investor to participate in a Proposed Transfer on the
terms and conditions specified in the Proposed Transfer Notice. 
 “Sale of the Corporation” shall have the meaning set
forth in Section 5.1. 
 “Selling Investors” shall have the meaning set forth in Section 5.2. 

“Senior Preferred” shall mean the Series E Preferred, Series D Preferred, Series C Preferred and the Series B-3 Preferred.

 “Senior Preferred Directors” shall have the meaning set forth in Section 4.1(a)(i). 

“Series A-3 Preferred” shall mean the Corporation’s Series A-3 Preferred Stock, $0.001 par value per share. 

“Series B-3 Preferred” shall mean the Corporation’s Series B-3 Preferred Stock, $0.001 par value per share. 

“Series C Preferred” shall have the meaning set forth in the Recitals. 

“Series D Preferred” shall have the meaning set forth in the Recitals. 

“Series E Preferred” shall have the meaning set forth in the Recitals. 

“Series E Purchase Agreement” shall have the meaning set forth in the Recitals. 

“Stock Sale” shall have the meaning set forth in Section 5.1. 

“Sun Mountain” shall have the meaning set forth in Section 4.1(a)(i)(A). 

“Sun Mountain Director” shall have the meaning set forth in Section 4.1(a)(i)(A). 

“Transferring Investor” shall have the meaning set forth in Section 3.1. 

“Tullis” shall have the meaning set forth in Section 4.1(a)(i)(B). 

“Tullis Director” shall have the meaning set forth in Section 4.1(a)(i)(B). 

“Vote” shall have the meaning set forth in Section 4.1. 

  
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 “Voting Securities” shall mean, with respect to an Investor, (i) all
Equity Securities and all other shares of capital stock and other securities of the Corporation now or hereafter owned of record, or beneficially, by such Investor, and (ii) all other Equity Securities and other shares of capital stock and
other securities over which the Investor has voting control. 
 SECTION 2. Preemptive Rights. 

2.1. Except for Excluded Securities, the Corporation shall not issue, sell or exchange, agree to issue, sell or exchange, or reserve or set
aside for issuance, sale or exchange, (i) any shares of Common Stock, (ii) any other equity security of the Corporation, (iii) any debt security of the Corporation which by its terms is convertible into or exchangeable for, with or
without consideration, any equity security of the Corporation, (iv) any security of the Corporation that is a combination of debt and equity or (v) any security convertible into, or exchangeable or exercisable for, shares of Common Stock,
or warrant or other right to subscribe for, purchase or otherwise acquire any equity security or any such debt security of the type described in clause (iii) or (iv) above of the Corporation (collectively, the “Financing
Securities”) unless in each case the Corporation shall have first offered to sell to each Investor then holding at least 10,000,000 shares in the aggregate of Series E Preferred, Series D Preferred and Series C Preferred (subject to
appropriate and proportionate adjustment for any stock dividends, stock splits and other subdivisions and combinations of, and recapitalizations and like occurrences with respect to, such shares) (or an equivalent amount of Common Stock issued upon
conversion thereof) (each, a “Major Investor” and collectively, the “Major Investors”) its pro-rata share of the Financing Securities, at a price and on such other terms as the Corporation proposes to offer such
Financing Securities to other parties and which shall have been specified by the Corporation in writing (an “Offer”) delivered to the Major Investors, which Offer by its terms shall remain open and irrevocable for a period of
fifteen (15) days from the date the Offer is received by the Major Investors (or, if later, within five (5) days after the giving of any written notice of a material change in such Offer). As used in this Section 2.1, a Major
Investor’s “pro-rata share” shall be that amount of the Financing Securities that equals (x) the aggregate amount of Financing Securities to be issued, sold or exchanged, multiplied by (y) a fraction, (A) the
numerator of which is the number of shares of Common Stock issued or issuable upon the conversion of all shares of Preferred Stock held by such Major Investor immediately prior to the Offer and (B) the denominator of which is the number of
shares of Common Stock issued or issuable upon the conversion of all shares of Preferred Stock held by all Major Investors immediately prior to the Offer. The Offer will specify (i) the aggregate amount of Financing Securities to be issued,
sold or exchanged, (ii) the calculation of the Major Investor’s pro rata share and (iii) the number of shares, principal amount or the like of the Financing Securities which such Major Investor may purchase. 

2.2. Notice of a Major Investor’s intention to accept, in whole or in part, an Offer shall be evidenced by a writing signed by the Major
Investor and delivered to the Corporation at or prior to the end of the 15-day period commencing with the date the Offer is received by the Major Investor (or, if later, within 5 days after the giving of any written notice of a material change in
such Offer), setting forth such portion (specifying the number of shares, principal amount or the like) of the Financing Securities as the Major Investor elects to purchase (the “Notice of Acceptance”). If a Major Investor elects to
purchase its full pro rata share of Financing Securities (each, an “Electing Investor”), then such Electing Investor shall have a right 

  
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of over-allotment such that if any other Major Investor fails to purchase its pro rata share (the “Non-electing Investor”), such Electing Investor may purchase, on a pro rata
basis with other Electing Investors, the Non-electing Investor’s pro rata share. 
 2.3. The Corporation shall have 180 days from the
expiration of the foregoing 15-day period to sell all or any part of such Financing Securities as to which a Notice of Acceptance has not been given by the Major Investors (the “Remaining
Securities”) to any other persons or entities, but only upon terms and conditions in all material respects, including without limitation, per share price and interest rates, which are no more favorable, in the aggregate, to such other
persons or entities or less favorable to the Corporation than those set forth in the Offer. Upon the closing of the sale to such other persons or entities of all or any part of the Remaining Securities, which shall include payment of the purchase
price to the Corporation in accordance with the terms of the Offer, if the Major Investor has timely submitted a Notice of Acceptance, it shall purchase from the Corporation, and the Corporation shall sell to the Major Investor, the Financing
Securities in respect of which a Notice of Acceptance was delivered to the Corporation by the Preferred Investor, at the terms specified in the Offer. The purchase by the Major Investor of any Financing Securities is subject in all cases to the
preparation, execution and delivery by the Corporation and the Major Investor of a purchase agreement and other customary documentation relating to such Financing Securities as is reasonably satisfactory in form and substance to the Preferred
Investor and its counsel. 
 2.4. After the expiration of the 180-day period referred to in Section 2.2, any Financing Securities not
purchased by the Major Investors or by a person or entity in accordance with Section 2.2 may not be sold or otherwise disposed of until they are again offered to the Preferred Investors under the procedures specified in Sections 2.1, 2.2 and
2.2 hereof. 
 2.5. Each Major Investor may assign its rights under this Section 2 to its Affiliates. 

SECTION 3. Right of Co-Sale. 

3.1. If any Investor proposes to engage in a Proposed Transfer (“Transferring Investor”), such Investor shall provide to each
Investor who owns Series E Preferred, Series D Preferred or Series C Preferred (each, a “Co-Sale Investor”) a Proposed Transfer Notice. Each Co-Sale Investor may elect to exercise its Right of Co-Sale and participate in the Proposed
Transfer on a pro rata basis as set forth below and otherwise on the same terms and conditions specified in the Proposed Transfer Notice (provided that if the Proposed Transfer is a transfer of Common Stock and a Co-Sale Investor wishes to sell
Preferred Stock, the price set forth in the Proposed Transfer Notice shall be appropriately adjusted based on the conversion ratio of the Preferred Stock into Common Stock). Each Co-Sale Investor who desires to exercise its Right of Co-Sale must
give the Transferring Investor written notice to that effect within fifteen (15) days after receipt of the Proposed Transfer Notice, and upon giving such notice, such Investor shall be deemed to have effectively exercised the Right of Co-Sale.

 3.2. Each Co-Sale Investor who timely exercises such Co-Sale Investor’s Right of Co-Sale by delivering the written notice provided
for above in Section 3.1 

  
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may include in the Proposed Transfer all or any part of such Co-Sale Investor’s Capital Stock equal to the product obtained by multiplying (i) the aggregate number of shares of Capital
Stock subject to the Proposed Transfer by (ii) a fraction, (A) the numerator of which is the number of shares of Common Stock issued or issuable upon the conversion of all shares of Preferred Stock held by such Co-Sale Investor immediately
before consummation of the Proposed Transfer and (B) the denominator of which is the total number of shares of Common Stock issued or issuable upon the conversion of all shares of Preferred Stock held by all Co-Sale Investors immediately before
consummation of the Proposed Transfer, plus the number of shares of Common Stock and shares of Common Stock issuable upon the conversion of all shares of Preferred Stock held by the Transferring Investor immediately before consummation of the
Proposed Transfer. To the extent one or more Co-Sale Investors exercise a Right of Co-Sale, the number of shares of Capital Stock that the Transferring Investor may sell in the Proposed Transfer shall be correspondingly reduced. 

3.3. Each Co-Sale Investor who timely exercises its Right of Co-Sale shall deliver to the Transferring Investor, no later than fifteen
(15) days after such Co-Sale Investor’s exercise of the Right of Co-Sale, one or more stock certificates, properly endorsed for transfer to the prospective transferee, representing the number of shares of Common Stock that such Co-Sale
Investor elects to include in the Proposed Transfer or the number of shares of Preferred Stock that is at such time convertible into the number of shares of Common Stock that such Co-Sale Investor elects to include in the Proposed Transfer;
provided, however, that if the prospective transferee objects to the delivery of convertible Preferred Stock in lieu of Common Stock, such Co-Sale Investor shall first convert the Preferred Stock into Common Stock and deliver Common Stock as
provided above. The Corporation agrees to make any such conversion concurrent with and contingent upon the actual transfer of such shares to the prospective transferee. 

3.4. Each stock certificate that a Co-Sale Investor delivers to the Transferring Investor pursuant to this Section 3 will be transferred
to the prospective transferee against payment therefor in consummation of the sale thereof pursuant to the terms and conditions specified in the Proposed Transfer Notice and the purchase and sale agreement, and the Transferring Investor shall
concurrently therewith remit or direct payment to each Co-Sale Investor of the portion of the sale proceeds to which such Co-Sale Investor is entitled by reason of its participation in such sale. If any prospective transferee refuses to purchase
securities subject to the Right of Co-Sale from any Co-Sale Investor exercising its Right of Co-Sale hereunder, none of the Co-Sale Investors or the Transferring Investor may sell any Capital Stock to such prospective transferee unless and until,
simultaneously with such sale, such Transferring Investor purchases all Capital Stock subject to the Right of Co-Sale from the Co-Sale Investor exercising its Right of Co-Sale on the same terms and conditions as set forth in the Proposed Transfer
Notice. 
 3.5. If any Proposed Transfer is not consummated within sixty (60) days after receipt of the Proposed Transfer Notice, the
Transferring Investor(s) proposing the Proposed Transfer may not sell any Capital Stock unless they first comply in full with each provision of this Section 3. The exercise or election not to exercise any right by a Co-Sale Investor hereunder
shall not adversely affect such Co-Sale Investor’s right to participate in any other sales of Capital Stock subject to Section 3. 

  
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 SECTION 4. Board of Directors. 

4.1. Composition. Each Investor shall hold all Voting Securities subject to, and shall vote or consent pursuant to an action by
written consent of the stockholders of the Corporation (“Vote”) such Voting Securities in accordance with, the provisions of this Agreement. 

(a) Election of Directors. 

(i) Senior Preferred Directors. On all matters relating to the election of the class of directors designated in the Certificate as
being elected only by the holders of Senior Preferred, the Investors holding Senior Preferred, acting together as a single class, shall Vote all of their respective Voting Securities so as to elect as a member of the Board three (3) individuals
(the “Senior Preferred Directors”) nominated as follows: 
 (A) For so long as NMSIC Co-Investment Fund, L.P. or
Affiliates thereof (“Sun Mountain”) owns any shares of Senior Preferred or Common Stock, one (1) director designated by Sun Mountain (the “Sun Mountain Director”). The Sun Mountain Director shall initially be
Brian Birk. 
 (B) For so long as Tullis-Dickerson Capital Focus III, L.P. or Affiliates thereof (“Tullis”) owns any
shares of Senior Preferred or Common Stock, one (1) director designated by Tullis (the “Tullis Director”). The Tullis Director shall initially be James L.L. Tullis. 

(C) For so long as PCM/Exagen, L.P. or Affiliates thereof (“PCM/Hunt”) owns any shares of Senior Preferred or Common Stock,
one (1) director designated by PCM/Hunt (the “PCM/Hunt Director”). The PCM/Hunt Director shall initially be Ebetuel Pallares. 

(D) If either or both of Sun Mountain or PCM/Hunt is no longer entitled to designate the Sun Mountain Director or the PCM/Hunt Director, as
applicable, then the director(s) to be elected by the holders of the issued and outstanding shares of Senior Preferred in lieu of the Sun Mountain Director or the PCM/Hunt Director shall be reasonably acceptable to Tullis for so long as Tullis is
entitled to designate a Senior Preferred Director pursuant to Section 4.1(a)(i)(B). 
 (ii) Common Director. On all matters
relating to the election of the class of directors designated in the Certificate as being elected only by the holders of Common Stock, the Investors holding Common Stock shall Vote all of their respective Voting Securities so as to elect one
(1) director who shall be the Corporation’s then-current Chief Executive Officer (the “Common Director”). The Common Director initially shall be Ron Rocca. 

(iii) Independent Directors. On all matters relating to the election of members of the Board other than the Senior Preferred Directors
and the Common Director, the Investors shall Vote all of their respective Voting Securities so as to elect two (2) directors nominated by Senior Preferred Directors, both of whom shall be independent and not

  
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 an Affiliate of the Corporation or any Investor and who shall have industry experience (the “Independent
Directors”). The Independent Directors initially shall be Frank Witney and John Radak. 
 (iv) Additional Director. On all
matters relating to the election of members of the Board other than the Senior Preferred Directors, the Common Director and the Independent Directors, the Investors agree to Vote all of their respective Voting Securities so as to elect one
(1) director designated by the holders of at least 52% of the then issued and outstanding shares of Senior Preferred, who shall initially be Arthur Weinstein. 

The obligation to Vote shares in accordance with this Section 4 shall be specifically applicable to and enforceable against any transferees of the
parties hereto. 
 (b) Vacancies; Removal. Each of the directors elected pursuant to Section 4 shall hold office, subject to
his or her resignation or earlier removal from the Board in accordance with the following sentence, in accordance with the Certificate, the Bylaws of the Corporation and applicable law, until his or her successors shall have been elected and shall
have been duly qualified. No Investor shall vote to remove any director elected pursuant to this Section 4, or to fill any vacancy created by the resignation or removal of a director elected pursuant to this Section 4, unless such action
shall have been approved by the Investors entitled to nominate and elect such director in accordance with the provisions of the Certificate and this Section 4. 

(c) Meetings. The Board shall hold meetings no less frequently than quarterly as determined by a majority of the Board. 

(d) Committees of the Board. The Corporation shall establish and maintain a compensation committee and an audit committee and may also
form an executive committee or any other special committee of the Board. Each of such committees shall have at least one (1) of the Senior Preferred Directors as a member of such committees. 

(e) Insurance. The Corporation shall obtain and maintain directors’ and officers’ liability insurance in reasonable amounts
from established and reputable insurers. 
 (f) Size of Board. Each Investor shall take all actions, do all things and execute and
deliver all documents, including, without limitation, approving an amendment to the Corporation’s Bylaws, and shall cause the Corporation to do the same, as may be necessary to ensure that the number of directors authorized and constituting the
entire Board shall be seven (7), subject to increase or decrease in accordance with the terms of the Certificate and pursuant hereto. 

(g) No Liability. No Investor, nor any Affiliate thereof, shall have any liability as a result of nominating a person for election as
a director, for any act or omission by such designated person in his or her capacity as a director of the Corporation, or as a result of voting for any such nominee in accordance with the provisions of this Agreement. 

  
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 (h) Irrevocable Proxy. Each party to this Agreement hereby grants to a stockholder
appointed by the Board, with full power of substitution, an irrevocable proxy with respect to the matters set forth herein, including, without limitation, the election of directors to the Board in accordance with Section 4.1(a) and
Section 4.1(b) hereof, and hereby authorizes such proxy to represent and to Vote, if and only if the party (i) fails to Vote, or (ii) attempts to Vote, in a manner which is inconsistent with the terms of this Agreement, all of such
party’s Voting Securities in favor of the election of directors to the Board determined pursuant to and in accordance with the terms and provisions of this Agreement. The proxy granted pursuant to the immediately preceding sentence is given in
consideration of the agreements and covenants of the parties in connection with the transactions contemplated by this Agreement and, as such, is coupled with an interest and shall be irrevocable unless and until this Agreement terminates pursuant to
the terms hereof. 
 SECTION 5. Drag-Along Right. 

5.1. A “Sale of the Corporation” shall mean either (a) a transaction or series of related transactions in which a
person, or a group of related persons, acquires from stockholders of the Corporation shares representing more than fifty percent (50%) of the outstanding voting power of the Corporation (a “Stock Sale”) or (b) a
transaction that is deemed to be a liquidation under Section 3.2 of the Certificate (a “Deemed Liquidation Event”). 

5.2. In the event that (i) Investors holding at least 52% of the shares of Senior Preferred then outstanding (collectively, the
“Selling Investors”) and (ii) the Board of Directors approve a Sale of the Corporation in writing, specifying that this Section 5 shall apply to such transaction, then each Investor hereby agrees: 

(a) if such transaction requires stockholder approval, with respect to all Capital Stock that such Investor owns or over which such Investor
exercises voting power, to vote, in person, by proxy, or by written consent, all Capital Stock in favor of, and adopt, such Sale of the Corporation (together with any related amendment to the Certificate required in order implement such Sale of the
Corporation) and to vote in opposition to any and all other proposals that could delay or impair the ability of the Corporation to consummate such Sale of the Corporation; 

(b) if such transaction is a Stock Sale, to sell the same proportion of shares of capital stock of the Corporation beneficially held by such
Investor as is being sold by the Selling Investors to the person to whom the Selling Investors propose to sell their Capital Stock, and, except as permitted in Section 5.2(c) below, on the same terms and conditions as the Selling Investors;

 (c) to execute and deliver all related documentation and take such other action in support of the Sale of the Corporation as shall
reasonably be requested by the Corporation or the Selling Investors in order to carry out the terms and provisions of this Section 5, including without limitation executing and delivering instruments of conveyance and transfer, and any purchase
agreement, merger agreement, indemnity agreement, escrow agreement, consent, waiver, governmental filing, share certificates duly endorsed for transfer 

  
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 (free and clear of impermissible liens, claims and encumbrances) and any similar or related documents; 

(d) not to deposit, and to cause their Affiliates not to deposit, except as provided in this Agreement, any Capital Stock owned by such
Investor or Affiliate in a voting trust or subject any Capital Stock to any arrangement or agreement with respect to the voting of such Capital Stock, unless specifically requested to do so by the acquiror in connection with the Sale of the
Corporation; 
 (e) to refrain from exercising any dissenters’ rights or rights of appraisal under applicable law at any time with
respect to such Sale of the Corporation; and 
 (f) if the consideration to be paid in exchange for the Capital Stock pursuant to this
Section 5 includes any securities and due receipt thereof by any Investor would require under applicable law (x) the registration or qualification of such securities or of any person as a broker or dealer or agent with respect to such
securities or (y) the provision to any Investor of any information other than information as a prudent issuer would generally furnish in an offering made solely to “accredited investors” as defined in Regulation D promulgated under
the Securities Act of 1933, as amended, the Corporation may cause to be paid to any such Investor in lieu thereof, against surrender of the Capital Stock which would have otherwise been sold by such Investor, an amount in cash equal to the fair
value (as determined in good faith by the Corporation) of the securities which such Investor would otherwise receive as of the date of the issuance of such securities in exchange for the Capital Stock. 

5.3. Notwithstanding the foregoing, an Investor will not be required to comply with Section 5.2(b) above in connection with any proposed
Sale of the Corporation (the “Proposed Sale”) unless: 
 (a) any representations and warranties to be made by such
Investor in connection with the Proposed Sale are limited to representations and warranties related to authority, ownership and the ability to convey title to such Capital Stock, including but not limited to representations and warranties that
(a) the Investor holds all right, title and interest in and to the Capital Stock such Investor purports to hold, free and clear of all liens and encumbrances, (b) the obligations of the Investor in connection with the transaction have been
duly authorized, if applicable, (c) the documents to be entered into by the Investor have been duly executed by the Investor and delivered to the acquirer and are enforceable against the Investor in accordance with their respective terms and
(d) neither the execution and delivery of documents to be entered into in connection with the transaction, nor the performance of the Investor’s obligations thereunder, will cause a breach or violation of the terms of any agreement, law or
judgment, order or decree of any court or governmental agency; 
 (b) the Investor shall not be liable for the inaccuracy of any
representation or warranty made by any other person in connection with the Proposed Sale, other than the Corporation (except to the extent that funds may be paid out of an escrow established to cover breach of representations, warranties and
covenants of the Corporation as well as breach by any Investor of any of identical representations, warranties and covenants provided by all Investors); 

  
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 (c) the liability for indemnification, if any, of such Investor in the Proposed Sale and for the
inaccuracy of any representations and warranties made by the Corporation or its Investors in connection with such Proposed Sale, is several and not joint with any other Person (except to the extent that funds may be paid out of an escrow established
to cover breach of representations, warranties and covenants of the Corporation as well as breach by any Investor of any of identical representations, warranties and covenants provided by all Investors), and is pro rata in proportion to, and does
not exceed, the amount of consideration paid to such Investor in connection with such Proposed Sale; 
 (d) liability shall be limited to
such Investor’s applicable share (determined based on the respective proceeds payable to each Investor in connection with such Proposed Sale in accordance with the provisions of the Certificate) of a negotiated aggregate indemnification amount
that applies equally to all Investors but that in no event exceeds the amount of consideration otherwise payable to such Investor in connection with such Proposed Sale, except with respect to claims related to fraud by such Investor, the liability
for which need not be limited as to such Investor; 
 (e) upon the consummation of the Proposed Sale, (i) each holder of each class or
series of the Corporation’s stock will receive the same form of consideration for their shares of such class or series as is received by other holders in respect of their shares of such same class or series of stock, (ii) each holder of a
series of Preferred Stock will receive the same amount of consideration per share of such series of Preferred Stock as is received by other holders in respect of their shares of such same series, (iii) each holder of Common Stock will receive
the same amount of consideration per share of Common Stock as is received by other holders in respect of their shares of Common Stock, and (iv) the aggregate consideration receivable by all holders of the Preferred Stock and Common Stock shall
be allocated among the holders of Preferred Stock and Common Stock on the basis of the relative liquidation preferences to which the holders of each respective series of Preferred Stock and the holders of Common Stock are entitled in a Deemed
Liquidation Event (assuming for this purpose that the Proposed Sale is a Deemed Liquidation Event) in accordance with the Certificate in effect immediately prior to the Proposed Sale; provided, however, that, notwithstanding the foregoing, if
the consideration to be paid in exchange for an Investor’s Capital Stock pursuant to this 5.2(e) includes any securities and due receipt thereof by any Investor would require under applicable law (x) the registration or qualification of
such securities or of any person as a broker or dealer or agent with respect to such securities or (y) the provision to any Investor of any information other than such information as a prudent issuer would generally furnish in an offering made
solely to “accredited investors” as defined in Regulation D promulgated under the Securities Act of 1933, as amended, the Corporation may cause to be paid to any such Investor in lieu thereof, against surrender of the Investor’s
Capital Stock which would have otherwise been sold by such Investor, an amount in cash equal to the fair value (as determined in good faith by the Corporation) of the securities which such Investor would otherwise receive as of the date of the
issuance of such securities in exchange for the Investor’s Capital Stock; and 
 (f) subject to the immediately preceding paragraph,
requiring the same form of consideration to be available to the holders of any single class or series of capital stock, if any holders of any capital stock of the Corporation are given an option as to the form and amount of consideration to be
received as a result of the Proposed Sale, all holders of 

  
 -12- 

 
such capital stock will be given the same option; provided, however, that nothing in this Section 5.2(f) shall entitle any holder to receive any form of consideration that such holder
would be ineligible to receive as a result of such holder’s failure to satisfy any condition, requirement or limitation that is generally applicable to the Corporation’s Investors. 

5.4. Proxy. Each Investor hereby grants to the Selling Investors (i) an irrevocable proxy, coupled with an interest, to Vote all
Voting Securities owned by such Investor, and (ii) an irrevocable power of attorney, which is coupled with an interest, to take such other actions, in each case to the extent necessary to carry out the provisions of this Section 5 in the
event of any breach by such Investor of its obligations hereunder. 
 SECTION 6. Miscellaneous. 

6.1. Voting Agreement. In the event that the taking of any one or more of the actions listed in Article Fourth, Section 7 of the
Certificate (an “Approved Action”) would require a separate vote by the holders of either of the Series A-3 Preferred or the Series B-3 Preferred under Section 242(b)(2) of the General Corporation Law of the State of Delaware,
then, in such event, each holder of Series A-3 Preferred and Series B-3 Preferred, as applicable, hereby agrees, with respect to all shares of Series A-3 Preferred or Series B-3 Preferred over which he, she or it exercises voting or dispositive
authority, to Vote all of such Series A-3 Preferred or Series B-3 Preferred in favor of the Approved Action if the holders of at least 50% of the then outstanding Senior Preferred, voting together as a single class, Vote to approve the taking of
such Approved Action. 
 6.2. Assignment of Rights. This Agreement and the rights and obligations of the parties hereunder shall
inure to the benefit of, and be binding upon, the parties’ respective successors, assigns and legal representatives; provided, however, that the rights of the Investors hereunder are only assignable to a assignee or transferee
(i) who acquires all of the securities of the Corporation purchased by an Investor or at least ten percent (10%) of such class and series of securities or (ii) who is a partner or retired partner of a partnership, and it shall be a
requirement of such transfer that such assignee shall then become a party to this Agreement. 
 6.3. Term. This Agreement shall
terminate upon the earlier of (i) the closing of a firmly underwritten initial public offering of the Corporation’s Common Stock or (ii) the acquisition of the Corporation by another entity by means of any transaction or series of
related transactions (including, without limitation, any reorganization, merger or consolidation) or a sale of all or substantially all of the assets of the Corporation, unless the Corporation’s stockholders of record as constituted immediately
prior to such acquisition or sale will, immediately after such acquisition or sale (by virtue of securities issued as consideration for the Corporation’s acquisition or sale or otherwise) hold at least 50% of the voting power of the surviving
or acquiring entity. 
 6.4. Waivers and Amendments. This Agreement may not be amended, modified or waived at any time, unless such
amendment, modification or waiver is first approved in writing by (a) the Investors owning at least 52% of the issued and outstanding shares of the Senior Preferred, voting together as a single class, and (b) the Corporation; provided,
however, that (i) if any amendment, modification or waiver would apply to a Holder (each, an “Adversely  

  
 -13- 

 
Affected Investor”) in a fashion different than how such amendment, modification or waiver applies to all Investors, then such amendment, modification or waiver shall not be effective
as to such Adversely Affected Investor unless consented to by such Adversely Affected Investor, (ii) any amendment, modification or waiver of Section 4.1(a)(i)(A), Section 4.1(b) (insofar as it relates to the Sun Mountain Director),
or of this clause (ii) shall require shall require the approval of Sun Mountain if at the time of such amendment, modification or waiver, Sun Mountain is entitled to designate a Senior Preferred Director pursuant to Section 4.1(a)(i)(A),
(iii) any amendment, modification or waiver of Section 4.1(a)(i)(B), Section 4.1(a)(i)(D), Section 4.1(b) (insofar as it relates to the Tullis Director), or of this clause (iii) shall require shall require the approval of
Tullis if at the time of such amendment, modification or waiver, Tullis is entitled to designate a Senior Preferred Director pursuant to Section 4.1(a)(i)(B), and (iv) any amendment, modification or waiver of Section 4.1(a)(i)(C),
Section 4.1(b) (insofar as it relates to the PCM/Hunt Director) or of this clause (iv) shall require shall require the approval of PCM/Hunt if at the time of such amendment, modification or waiver, PCM/Hunt is entitled to designate a
Senior Preferred Director pursuant to Section 4.1(a)(i)(C). Any amendment, modification or waiver so effected shall be binding upon the Corporation, each of the parties hereto and any assignee of any such party. In addition, the Corporation may
waive performance of any obligation owing to it, as to some or all of the Investors, or agree to accept alternatives to such performance, without obtaining the consent of any Investor. No waiver of any breach of this Agreement extended by any party
hereto to any other party shall be construed as a waiver of any rights or remedies of any other party hereto or with respect to any subsequent breach 

6.5. Governing Law. This Agreement shall be governed in all respects by the laws of the State of Delaware, without regards to conflict
of laws principles. 
 6.6. Effectiveness; Entire Agreement. This Agreement shall become effective concurrently with the
consummation of the purchase and sale of the Series E Preferred under the Series E Purchase Agreement at the Initial Closing. This Agreement and the other documents delivered in connection herewith constitute the full and entire understanding of the
parties with regard to the subjects hereof and thereof, and this Agreement shall supersede and cancel all prior agreements between the parties hereto with regard to the subject matter hereof, including the Prior Agreement. 

6.7. Notices, etc. All notices and other communications required or permitted hereunder (each, a “Notice”) shall be
in writing and shall be delivered by overnight courier service or mailed by first class mail, postage prepaid, certified or registered mail, return receipt requested, addressed (a) if to an Investor, at such party’s address as set forth in
the Corporation’s records, or at such other address as such party shall have furnished to the Corporation in writing or (b) if to the Corporation, to it at the address set forth on the Corporation’s signature page hereto or other
address as the Corporation shall have furnished to the Investor in writing. All Notices shall be deemed effectively given: (x) upon personal delivery to the party to be notified, (y) three (3) days after having been sent by first
class mail, postage prepaid, certified or registered mail, return receipt requested and (z) one (1) day after deposit with an overnight courier service. 

  
 -14- 

 6.8. Severability. In case any provision of this Agreement shall be invalid, illegal or
unenforceable, the validity, legality and enforceability of the remaining provisions of this Agreement shall not in any way be affected or impaired thereby. 

6.9. Title and Subtitles. The titles of the sections and subsections of this Agreement are for convenience of reference only and are
not to be considered in construing this Agreement. 
 6.10. Attorneys’ Fees. If any action at law or in equity is necessary to
enforce or interpret the terms of this Agreement, the prevailing party shall be entitled to reasonable attorneys’ fees, costs and disbursements in addition to any other relief to which such party may be entitled. 

6.11. Counterparts; Execution by Facsimile Signature. This Agreement may be executed in any number of counterparts, each of which
shall be an original, but all of which together shall constitute one instrument. This Agreement may be executed by facsimile signature(s) which shall be binding on the party delivering same. 

6.12. Aggregation of Stock. All shares of Preferred Stock and Common Stock of the Corporation held or acquired by affiliated entities
or persons shall be aggregated for the purpose of determining the availability of any rights under this Agreement. 
 6.13. Additional
Investors. Notwithstanding anything to the contrary contained herein, if the Corporation shall issue additional shares of Preferred Stock from time to time, any purchaser of such shares of Preferred Stock may become a party to this Agreement by
executing and delivering an additional counterpart signature page to this Agreement and shall be deemed an “Investor” and a party hereunder. 

6.14. Specific Performance. The parties hereto hereby declare that it is impossible to measure in money the damages which will accrue
to a party hereto or to their heirs, personal representatives, successors or assigns by reason of the failure of a party to perform any of the obligations under this Agreement and agree that the terms of this Agreement shall be specifically
enforceable. If any party hereto or such party’s heirs, personal representatives, or assigns institutes any action or proceeding to specifically enforce the provisions hereof, any person against whom such action or proceeding is brought hereby
waives the claim or defense therein that such party or such personal representative has an adequate remedy at law, and such person shall not offer in any such action or proceeding the claim or defense that such remedy at law exists. 

  *  *  *  *  *  *  * 

  
 -15- 

 IN WITNESS WHEREOF, the parties hereto have executed this Fifth Amended and Restated
Stockholders Agreement as of the date first above written. 

			
	
	CORPORATION:
	
	EXAGEN DIAGNOSTICS, INC.
		
	By:	 	 /s/ Fortunato Ron Rocca

	Name:	 	Fortunato Ron Rocca
	Title:	 	President and Chief Executive Officer

 
			
		
	Address:	 	1261 Liberty Way, Suite C
		 	Vista, CA 92081

  
 [Signature Page to Fifth
Amended and Restated Stockholders Agreement] 

 
			
	INVESTORS:
	
	WASATCH VENTURE FUND III, LLC
		
	By:	 	 /s/ Nick Efstratis

	Name:	 	Nick Efstratis
	Title:	 	Managing Director
	
	WASATCH NEW MEXICO FUND, LLC
		
	By:	 	 /s/ Nick Efstratis

	Name:	 	Nick Efstratis
	Title:	 	Managing Director
	
	NMSIC FOCUSED LLC
	
	By: NMSIC Co-Investment Fund LP, its sole member
	By: Sun Mountain Capital Partners LLC, its general partner
		
	By:	 	 /s/ Brian Birk

		 	Brian Birk, Managing Partner
		
	By:	 	 /s/ Lee Rand

		 	Lee Rand, Managing Member
	
	 JAMES J. SCHWARZ and JEANETTE M.

SCHWARZ, CO-TRUSTEES OF THE
 SCHWARZ
TRUST

		
	By:	 	 /s/ James J. Schwarz

		 	James J. Schwarz
	
	NEW TECH I, L.P.
		
	By:	 	 /s/ David Durgin

	Name:	 	David Durgin

  
 [Signature Page to Fifth
Amended and Restated Stockholders Agreement] 

 
			
	Title: General Partner
	
	QUATRO VENTURES, LLC
		
	By:	 	 /s/ David Durgin

	Name:	 	David Durgin
	Title:	 	General Partner
	
	 /s/ Samuel D. Riccitelli

	SAMUEL D. RICCITELLI
	
	SOUTHWEST MEDICAL VENTURES, INC.
		
	By:	 	 /s/ Waneta Tuttle

	Name:	 	Waneta C. Tuttle
	Title:	 	President
	
	TULLIS-DICKERSON CAPITAL FOCUS III, L.P.
		
	By:	 	Tullis-Dickerson Partners III, L.L.C., its general partner
		
	By:	 	 /s/ James L.L. Tullis

	Name:	 	James L.L. Tullis
	Title:	 	Manager
	
	TULLIS GROWTH FUND, L.P.
		
	By:	 	Tullis Growth Partners, L.L.C., its general partner
		
	By:	 	 /s/ James L.L. Tullis

	Name:	 	James L.L. Tullis
	Title:	 	Manager

 [Signature Page to Fifth Amended and Restated Stockholders Agreement] 

 
			
	HUNT HOLDINGS LIMITED PARTNERSHIP
	By:	 	HuntVest, LLC, its General Partner
	By:	 	Hunt Guaranty Inc., its Sole Member
		
	 By:
	 	 /s/ Matthew Hunt

	 Name:
	 	Matthew D. Hunt
	 Title:
	 	Managing Partner
	
	PCM/EXAGEN, L.P.
		
	 By:
	 	 /s/ Ebetuel Pallares

	 Name:
	 	Ebetuel Pallares
	 Title:
	 	Managing Partner
	
	CCP/EXAGEN, L.P.
		
	 By:
	 	 /s/ Ebetuel Pallares

	 Name:
	 	Ebetuel Pallares
	 Title:
	 	Managing Partner

 [Signature Page to Fifth Amended and Restated Stockholders Agreement] 

 SCHEDULE A 

INVESTORS 
 Wasatch Venture Fund
III, LP 
 Wasatch New Mexico Fund, LLC 
 vSpring SBIC, L.P.

 vSpring, L.P. 
 vSpring Partners, L.P. 

NMSIC Co-Investment Fund, L.P. 
 NMSIC Focused, LLC 

James J. Schwarz and Jeanette M. Schwarz, Co-Trustees of the Schwarz Trust 

Jane Hillhouse 
 Lisa Davis 

New Tech I, L.P. 
 Pamela J. Sullivan and Thomas A. Tumolillo 

Quatro Ventures, LLC 
 Ray Radosevich 

Robert and Marcia Cates 
 Robert H. Nath 

Southwest Medical Ventures, Inc. 
 Sheryl A. Johnson 

John P. Alsobrook, II 
 Dale Olson 

Mesa Verde Venture Partners, L.P. 
 PCM/Exagen, L.P. 

CCP/Exagen, L.P. 
 Hunt Holdings L.P. 

Tullis-Dickerson Capital Focus III, L.P. 
 Tullis Growth Fund,
L.P. 
 Berge K. Hagopian and Mary Ann Hagopian, Co-Trustees, Hagopian Family Trust UA DTD 03/25/88 

Timothy M. Pennington and Melissa J. Pennington as Trustees of the Pennington Family Revocable Trust UA Dated May 23, 1984 

Endochoice, Inc. 
 Glenn Holdings, L.P. 

  
 A-1EX-4.8

 Exhibit 4.8 

THIS WARRANT AND THE SECURITIES ISSUABLE UPON THE EXERCISE HEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES
ACT”), OR ANY STATE SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED, HYPOTHECATED OR OTHERWISE TRANSFERRED IN THE ABSENCE OF A REGISTRATION STATEMENT IN EFFECT WITH RESPECT TO THE SECURITIES UNDER THE SECURITIES ACT OR AN
OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED OR UNLESS SOLD PURSUANT TO AN EXEMPTION TO THE SECURITIES ACT. 

THE SALE OF THE SECURITIES WHICH ARE THE SUBJECT OF THIS WARRANT HAS NOT BEEN QUALIFIED WITH THE COMMISSIONER OF CORPORATIONS OF THE STATE OF CALIFORNIA OR
ANY OTHER STATE AND THE ISSUANCE OF SUCH SECURITIES OR THE PAYMENT OR RECEIPT OF ANY PART OF THE CONSIDERATION FOR SUCH SECURITIES PRIOR TO SUCH QUALIFICATION IS UNLAWFUL, UNLESS THE SALE OF SUCH SECURITIES IS EXEMPT FROM QUALIFICATION BY SECTION
2511, 25102 OR 25105 OF THE CALIFORNIA CORPORATIONS CODE OR SUCH PROVISIONS OF THE CORPORATIONS CODE OF ANY SUCH OTHER STATE. THE RIGHTS OF THE HOLDER OF THIS WARRANT ARE EXPRESSLY CONDITIONED UPON SUCH QUALIFICATION BEING OBTAINED, UNLESS THE SALE
IS SO EXEMPT. 
 Void after 

October [    ], 2020 

WARRANT TO PURCHASE SHARES 

OF PREFERRED STOCK 
 of

 EXAGEN DIAGNOSTICS, INC. 

INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE 

THIS CERTIFIES THAT, for value received,
                                , together with its permitted successors
and assigns (“Holder”) is entitled, subject to the terms set forth below, to subscribe for and purchase shares of a series of Preferred Stock (the “Preferred Stock”) of Exagen Diagnostics, Inc., a
Delaware corporation (the “Company”), subject to adjustment as provided herein. This warrant and any warrant subsequently issued upon exchange or transfer hereof are hereinafter referred to collectively as the
“Warrant.” This Warrant is one of a series of warrants issued pursuant to that certain Note and Warrant Purchase Agreement dated October [ ], 2015 by and among the Company and the entities and persons listed on the Schedule
of Investors thereto (the “Purchase Agreement”), and the Holder and the Company shall be bound by all the terms, conditions and provisions of the Purchase Agreement. 

This Warrant is subject to the following terms and conditions: 

1. Convertible Promissory Note. This Warrant is issued in connection with that certain Convertible Promissory Note dated October [ ],
2015 (the “Note”) by the Company in favor of Holder. All capitalized terms used but not defined in this Warrant shall have the meanings ascribed thereto in the Note. 

  
 1 

 2. Exercise of Warrant. The terms and conditions upon which this Warrant may be exercised,
and the shares covered hereby may be purchased, are as follows: 
 2.1 Term. Subject to the terms hereof and unless sooner terminated
as provided below in Section 7.3, this Warrant may be exercised at any time after the date hereof, or from time to time, in whole or in part; provided, however, that in no event may this Warrant be exercised (the “Exercise
Date”) later than 5:00 p.m. (Pacific Time) on the close of business on October [ ], 2020 (the “Exercise Period”). 

2.2 Number of Shares. This Warrant may be exercised for a number of Shares, as defined below, as set forth below: 

2.2.1 Equity Financing. In the event (a) the Company completes a Qualified Equity Financing (as defined below) or (b) the
Note is converted into equity securities of the Company in accordance with Sections 4(b) or 4(c) of the Note, the number of Shares subject to this Warrant will be equal to the following: 

 

					
	A =	 	.20(B)	 	
		 	    C	 	
	Where:        	 		 	
		 	A  =	 	The number of Shares that may be purchased by Holder pursuant to this Warrant.
			
		 	B  =	 	The original principal amount of the Note held by Holder.
			
		 	C  =	 	The Exercise Price (as defined below) for the Shares.

 The term “Qualified Equity Financing” shall mean an equity financing after the date
hereof which results in aggregate gross proceeds to the Company of at least Ten Million Dollars ($10,000,000), excluding the conversion of the Notes, and in which investors purchase shares of a newly authorized series of the Company’s Preferred
Stock and in which the Notes are converted into Shares. 
 The term “Shares” shall mean (a) if a Qualified
Equity Financing occurs, the shares of Preferred Stock sold in the Qualified Equity Financing (the “New Preferred Stock”), or (b) if the Note has been converted into the Company’s Series D Preferred Stock, par value
$0.001 per share (the “Series D Preferred Stock”), pursuant to Sections 4(b)(i), 4(b)(ii)(A), 4(b)(iii) or 4(c) of the Note, such Series D Preferred Stock, in each case, subject to further adjustment as provided herein. The
Company agrees to use commercially reasonable efforts to obtain the requisite approvals to authorize sufficient Series D Preferred Stock in the event the Shares issuable upon exercise of this Warrant are shares of Series D Preferred Stock. 

2.3 Exercise Price. The “Exercise Price” shall be equal to (a) in case that the Shares being purchased
upon exercise of this Warrant are shares of New Preferred Stock, the lowest price per share paid by investors for such New Preferred Stock and (b) in the case that the Shares being purchased upon exercise of this Warrant are shares of Series D
Preferred Stock, the then applicable Conversion Price (as defined in the Company’s Fifteenth Amended and Restated Certificate of Incorporation, as the same may be amended from time to time), in each case, subject to adjustment as provided
herein. 

  
 2 

 2.4 Method of Exercise. Subject to the terms and conditions contained herein and while
this Warrant remains outstanding and exercisable, from and after the Qualified Equity Financing or conversion of the Note, as applicable, this Warrant is exercisable with respect to any or all of the Shares, at the option of Holder, upon surrender
of this Warrant to the Company together with (a) a duly completed (i) Notice of Exercise, in the form attached hereto as Exhibit A, or (ii) Net Issue Election Notice, in the form attached hereto as Exhibit B and
(b) payment of an amount equal to the Exercise Price multiplied by the number of Shares with respect to which this Warrant is being exercised as provided in Section 2.5 below. If Holder exercises this Warrant with respect to less than all
of the Shares represented by this Warrant, the Company shall cancel this Warrant upon the surrender thereof and shall execute and deliver to Holder a new Warrant for the balance of such Shares. 

2.5 Payment. Payment of the Exercise Price for the Shares with respect to which this Warrant is being exercised by Holder shall be
made, at the option of Holder, (a) by delivery of cash payable by wire transfer of immediately available funds, (b) by the delivery of a cashier’s check or certified check, (c) by net issue election as set forth in
Section 2.6 below, or (d) by any combination of (a) – (c). 
 2.6 Net Issue Election Holder may elect to
receive, without payment by Holder of any additional consideration, Shares equal to the value of the “spread” on the Shares or any portion thereof by the surrender of the Warrant to the Company, together with a duly completed Net Issue
Election Notice, in the form attached hereto as Exhibit B, at the principal office of the Company, in which event the Company shall issue to Holder such number of Shares as is computed using the following formula: 

 

					
	X =	 	Y (A – B)	 	
		 	      A	 	

					
			
	Where:        	 	X  =	 	The number of Shares to be issued to Holder pursuant to the net issue election;
			
		 	Y  =	 	The number of Shares in respect of which the net issue election is made;
			
		 	A  =	 	The fair market value (as determined below) of one Share at the time the net issue election is made;
			
		 	B  =	 	The Exercise Price in effect under this Warrant as of the date of the net issue election.

 For purposes of this Section 2.6, the fair market value of one Share as of a particular date shall be as determined in
good faith by the Board of Directors of the Company. 
 3. Adjustment of Exercise Price and Number of Shares. The Exercise Price and
the number and kind of Shares purchasable upon the exercise of this Warrant shall be subject to adjustment from time to time upon the happening of certain events as follows: 

3.1 Conversion of Shares into Common Stock. Upon conversion of all of the issued and outstanding shares of the Company’s Preferred
Stock into Common Stock (“Common Stock”), this Warrant shall be automatically exercisable only for such number of shares of Common Stock as Holder would have received had this Warrant been exercised in full for the Shares and
then converted into Common Stock on the date all issued and outstanding shares of the Company’s Preferred Stock converted into Common Stock. The Exercise Price in effect immediately prior to such conversion shall,

  
 3 

 
concurrently with the effectiveness of such conversion, be proportionally adjusted. Upon such conversion of the Preferred Stock into Common Stock, all references under this Warrant to Shares
shall be deemed references to Common Stock. 
 3.2 Split, Subdivision or Combination. If the Company should at any time or from time
to time fix a record date for (a) the effectuation of a split or subdivision of the outstanding Shares or (b) the determination of Holders of Shares entitled to receive a dividend or other distribution payable in additional Shares or other
securities or rights convertible into, or entitling Holder thereof to receive directly or indirectly, additional Shares (hereinafter referred to as the “Share Equivalents”), without payment of any consideration by such holder
for the additional Shares or Share Equivalents, then, as of such record date (or the date of such distribution, split or subdivision if no record date is fixed), the Exercise Price shall be appropriately decreased and the number of Shares which this
Warrant is exercisable for, if any, shall be appropriately increased in proportion to such increase of outstanding shares. Notwithstanding the foregoing, in any such case, the aggregate purchase price payable by Holder for the total number of Shares
(as adjusted) shall remain the same. 
 3.3 Combination of Shares. If the number of Shares outstanding at any time after the date
hereof is decreased by a combination of the outstanding Shares, the Exercise Price shall be appropriately increased and the number of Shares for which this Warrant is exercisable, if any, shall be appropriately decreased in proportion to such
decrease in outstanding shares. Notwithstanding the foregoing, in any such case, the aggregate purchase price payable by Holder for the total number of Shares (as adjusted) shall remain the same. 

3.4 Reclassification or Reorganization. If the Shares shall be changed into the same or different number of shares of any class or
classes of stock, whether by capital reorganization, reclassification or otherwise (other than a subdivision, conversion or combination of shares or stock dividend provided for in Sections 3.1, 3.2 and 3.3 above), then and in each such event Holder
shall be entitled to receive upon the exercise of this Warrant the kind and amount of shares of stock and other securities and property receivable upon such reorganization, reclassification or other change, to which a holder of the number of Shares
(or any shares of stock or other securities which may be) issuable upon the exercise of this Warrant would have received if this Warrant had been exercised immediately prior to such reorganization, reclassification or other change, all subject to
further adjustment as provided herein. At the request of Holder, this Warrant will thereupon be cancelled and upon its surrender to the Company, the Company will execute and deliver at its expense a new Warrant reflecting the foregoing adjustment,
but otherwise identical to the replaced Warrant. 
 3.5 Notice of Adjustments and Record Dates. The Company shall promptly notify
Holder in writing of each adjustment or readjustment of the Exercise Price hereunder and the number of Shares issuable upon the exercise of this Warrant. Such notice shall state the adjustment or readjustment and show in reasonable detail the facts
on which that adjustment or readjustment is based. In the event of any taking by the Company of a record of holders of Shares for the purpose of determining holders thereof who are entitled to receive any dividend or other distribution, the Company
shall notify Holder in writing of such record date at least twenty (20) days prior to the date specified therein. 
 3.6 Fractional
Shares. No fractional shares shall be issued upon the exercise of this Warrant as a consequence of any adjustment pursuant hereto. All Shares (including fractions) issuable upon exercise of this Warrant may be aggregated for purposes of
determining whether the exercise would result in the issuance of a fractional share. If, after aggregation, the exercise would result in the issuance of a fractional share, the Company shall, in lieu of issuance of any fractional share, pay Holder
otherwise entitled to such fraction a sum in cash equal to the product resulting from multiplying the then current fair market value of a Share by such fraction. 

  
 4 

 3.7 Issue Tax. The issuance of certificates for the Shares upon exercise of this Warrant
shall be made without charge to Holder for any issuance tax 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 that of Holder. 
 3.8 No Impairment. The Company shall not avoid or seek to avoid the observance or performance of
any of the terms to be observed or performed hereunder by the Company, but shall at all times in good faith assist in the carrying out of all the provisions of this Warrant. Without limiting the generality of the foregoing, the Company shall take
all such action as may be necessary or appropriate in order that all shares of Shares as may be issued pursuant to the exercise of this Warrant shall, upon issuance, be duly and validly issued, fully paid and nonassessable and free from all taxes,
liens and charges with respect to the issue thereof. 
 4. Replacement of Warrants. On receipt by the Company 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 of this Warrant, on delivery of an indemnity agreement reasonably satisfactory in form and amount to
the Company or, in the case of any such mutilation, on surrender and cancellation of such Warrant, the Company at its expense shall execute and deliver to Holder, in lieu thereof, a new Warrant of like tenor. 

5. No Rights or Liability as a Stockholder. This Warrant does not entitle Holder hereof to any voting rights or other rights as a
stockholder of the Company. No provisions hereof, in the absence of affirmative action by Holder to purchase Shares, and no enumeration herein of the rights or privileges of Holder, shall give rise to any liability of Holder as a shareholder of the
Company. 
 6. Registration Rights. The Shares issuable upon exercise of this Warrant shall be entitled to registration rights and
all other rights as applicable to Common Stock issuable upon conversion of New Equity Shares under any investors’ rights agreement entered into between the Company and the holders of New Equity Shares. 

7. Miscellaneous. 
 7.1
Limitations on Disposition. Holder agrees not to make any disposition of this Warrant or any Shares, unless and until (i) the transferee has agreed in writing for the benefit of the Company to be bound by this Section 7.1 and the
other provisions of this Warrant as if such transferee were the original Holder hereof, provided and to the extent such provisions are then applicable, and (ii) (A) there is then in effect a registration statement under the Securities Act
of 1933, as amended (the “Securities Act”), covering such proposed disposition and such disposition is made in accordance with such registration statement, or (B) Holder shall have notified the Company of the proposed
disposition and shall have furnished the Company with a detailed description of the manner and circumstances of the proposed disposition, and, if requested by the Company, such Holder shall have furnished the Company, at its expense, with either
(x) an opinion of counsel, reasonably satisfactory to the Company, to the effect that such disposition will not require registration of such Securities under the Securities Act or (y) a “no action” letter from the Securities and
Exchange Commission to the effect that the transfer of such securities without registration will not result in a recommendation by the staff of the Securities and Exchange Commission that action be taken with respect thereto, whereupon the holder of
such Securities shall be entitled to transfer such Securities in accordance with the terms of the notice delivered by Holder to the Company. It is agreed that the Company will not require opinions of counsel for transactions made pursuant to Rule
144 under the Securities Act except in unusual circumstances. 

  
 5 

 7.2 Permitted Transfers. Notwithstanding the provisions of Section 7.1 above, no such
registration statement, prior consent or opinion of counsel shall be necessary for (i) a transfer not involving a change in beneficial ownership, or (ii) in transactions involving the distribution without consideration by any Holder to
(x) a parent, subsidiary or other affiliate of Holder that is a corporation, (y) any of its partners, members or other equity owners, or retired partners, retired members or other equity owners, or to the estate of any of its partners,
members or other equity owners or retired partners, retired members or other equity owners, or (z) a venture capital fund that is controlled by or under common control with one or more general partners or managing members of, or shares the same
management company with, such Holder, or (iii) transfers in compliance with Rule 144 under the Securities Act, as long as the Company is furnished with satisfactory evidence of compliance with such Rule; provided, in each case, that Holder
thereof shall give written notice to the Company of such Holder’s intention to effect such disposition and shall have furnished the Company with a detailed description of the manner and circumstances of the proposed disposition. Each new
certificate evidencing this Warrant and/or the Shares so transferred shall bear the appropriate restrictive legends, except that such certificate shall not bear such restrictive legend if, in the opinion of counsel for the Company, such legend is
not required in order to establish or assist in compliance with any provisions of the Securities Act or any applicable state securities laws. 

7.3 Early Termination. In the event of, at any time during the Exercise Period, an initial public offering of securities of the Company
registered under the Securities Act, or any capital reorganization, or any reclassification of the capital stock of the Company (other than a change in par value or from par value to no par value or no par value to par value or as a result of a
stock dividend or subdivision, split-up or combination of shares), or the consolidation or merger of the Company with or into another corporation (other than a merger solely to effect a reincorporation of the Company into another state), or the sale
or other disposition of all or substantially all the properties and assets of the Company in its entirety to any other person, the Company shall provide to Holder ten (10) days advance written notice of such public offering, reorganization,
reclassification, consolidation, merger or sale or other disposition of the Company’s assets, and this Warrant shall terminate unless exercised prior to the date such public offering is closed or the occurrence of such reorganization,
reclassification, consolidation, merger or sale or other disposition of the Company’s assets. 
 8. Titles and Subtitles. The
titles and subtitles used in this Warrant are for convenience only and are not to be considered in construing or interpreting this Warrant. 

9. Notices. All notices and other communications under this Warrant shall be in writing and shall be deemed given upon receipt if
delivered personally, or when sent if mailed by registered or certified mail (return receipt requested) or by reputable overnight express courier (charges prepaid) or transmitted by facsimile (with confirmation of transmittal) to the party to be
notified at the address indicated for such party on the signature page hereof, or at such other address as such party may designate by advance written notice to the other parties. 

10. Attorneys’ Fees. If any action at law or in equity is necessary to enforce or interpret the terms of this Warrant, the
prevailing party shall be entitled to reasonable attorneys’ fees, costs and disbursements in addition to any other relief to which such party may be entitled. 

11. Amendments and Waivers. This Warrant may be amended and the observance of any other term of this Warrant may be waived (either
generally or in a particular instance and either retroactively or prospectively), with the written consent of the Company and the holders of a majority in interest of the Shares issuable upon exercise of the Warrants issued pursuant to the Purchase
Agreement. Any amendment or waiver effected in accordance with this Section 11 shall be binding upon Holder of 

  
 6 

 
this Warrant (and of any Shares into which this Warrant is exercisable), and each future holder of all such securities and the Company. 

12. Severability. If one or more provisions of this Warrant are held to be unenforceable under applicable law, such provision shall be
excluded from this Warrant and the balance of the Warrant shall be interpreted as if such provision were so excluded and shall be enforceable in accordance with its terms. 

13. Governing Law. This Warrant shall be governed by and construed and enforced in accordance with the laws of the State of Delaware,
without giving effect to its conflicts of laws principles. 
 [SIGNATURE PAGE FOLLOWS] 

  
 7 

 This Warrant may be executed in one or more counterparts, each of which shall be deemed an
original, but all of which together shall constitute one and the same instrument. 
  

							
	Date: October [    ], 2015	 		 	EXAGEN DIAGNOSTICS, INC.
		 		 	a Delaware corporation
				
		 		 	By:	 	  

		 		 	Name:	 	Fortunato Ron Rocca
		 		 	Title:	 	Chief Executive Officer
				
		 		 	Address:	 	1261 Liberty Way
		 		 		 	Vista, CA 92081

  

			
	ACKNOWLEDGED AND AGREED:
	
	[HOLDER]
		
	By:	 	 
		
	Name:	 	  

		
	Title:	 	  

  

			
		
	Address:	 	  

		 	  

		 	  

		 	Fax:                                     
                       

 EXAGEN DIAGNOSTICS, INC. 

WARRANT TO PURCHASE SHARES OF 

PREFERRED STOCK 

 EXHIBIT A 

FORM OF NOTICE OF EXERCISE 
 The
undersigned, the holder of the within Warrant, hereby irrevocably elects to exercise this Warrant for, and to purchase thereunder,
                     Shares (as defined in the attached Warrant)* of EXAGEN DIAGNOSTICS,
INC., a Delaware corporation and herewith makes payment of $                     therefor and requests that the
certificates for such shares be issued in the name of, and delivered to,
                                , federal taxpayer identification number
                    , whose address is
                                         
                   . 
 In exercising this
Warrant, the undersigned hereby confirms and acknowledges that the                      Shares (as defined in the attached Warrant) are being
acquired solely for the account of the undersigned and not as a nominee for any other party, and for investment, and the undersigned will not offer, sell or otherwise dispose of any such Shares except under circumstances that will not result in a
violation of the Securities Act of 1933, as amended, or any state securities laws. 
 Please issue a new Warrant for the unexercised portion
of the attached Warrant in the name of, and delivered to,
                                , federal taxpayer identification number
                    , whose address is
                                         
                       . 

Dated:                     

 

	
	
	  

	(Signature must conform to name of holder as specified on the face of the Warrant)

  

	*	Insert here the number of shares as to which the Warrant is being exercised. 

 EXHIBIT B 

FORM OF NET ISSUE ELECTION NOTICE 

(To be signed only on net issue exercise of the Warrant) 

The undersigned, the holder of the within Warrant, hereby irrevocably elects to exercise this Warrant with respect to
                     Shares (as defined in the attached Warrant) of EXAGEN DIAGNOSTICS,
INC., a Delaware corporation, pursuant to the net issue election provisions set forth in Section 2.6 of the Warrant and requests that the certificates for the number of Shares issuable pursuant to said Section 2.6 after
application of the net issue election formula to such Shares be issued in the name of, and delivered to,
                                , federal taxpayer identification number
                    , whose address is
                                         
                   . 
 In exercising this
Warrant, the undersigned hereby confirms and acknowledges that the Shares are being acquired solely for the account of the undersigned and not as a nominee for any other party, and for investment, and the undersigned will not offer, sell or
otherwise dispose of any such Shares except under circumstances that will not result in a violation of the Securities Act of 1933, as amended, or any state securities laws. 

Please issue a new Warrant for the unexercised portion of the attached Warrant in the name of, and delivered to,
                                , federal taxpayer identification number
                    , whose address is
                                         
                   . 

Dated:                     

 

	
	  

	(Signature must conform to name of holder as specified on the face of the Warrant)

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