Description of Securities Registered Under Section 12 of the Exchange Act

The following description of the common stock, par value $0.001 per share (the “Common Stock”) of Infinity Pharmaceuticals, Inc. (“us,” “our,” “we” or the “Company”), which is the only security of the Company registered under Section 12 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), summarizes certain information regarding the Common Stock in our Restated Certificate of Incorporation (the “Certificate of Incorporation”), our Amended and Restated Bylaws (the “Bylaws”) and applicable provisions of Delaware General Corporation Law (the “DGCL”), and is qualified by reference to the Certificate of Incorporation and the Bylaws, which are incorporated by reference as Exhibit 3.1 and Exhibit 3.2, respectively, to the Annual Report on Form 10-K of which this Exhibit 4.2 is a part.

Our authorized capital stock consists of 200,000,000 shares of Common Stock and 1,000,000 shares of preferred stock, par value $0.001 per share (the “Preferred Stock”).

Annual Meeting. Annual meetings of our stockholders are held on the date designated in accordance with our Bylaws. Written notice must be mailed to each stockholder entitled to vote not less than ten nor more than 60 days before the date of the meeting. The presence in person or by proxy of the holders of record of a majority of our issued and outstanding shares entitled to vote at such meeting constitutes a quorum for the transaction of business at meetings of the stockholders. Special meetings of the stockholders, unless otherwise prescribed by statute or by our Certificate of Incorporation, may only be called, for any purpose or purposes, by our chief executive officer, the chairman of our board of directors, or a majority of our board of directors. Except as may be otherwise provided by applicable law, our Certificate of Incorporation, our Bylaws or the rules and regulations of any stock exchange applicable to us, all elections, other than elections of directors, and all other questions shall be decided by the affirmative vote of the holders of a majority in voting power of the shares of our stock which are present in person or by proxy and entitled to vote thereon. Except as may be provided by applicable law, our Certificate of Incorporation, or our Bylaws, each director shall be elected by the vote of the majority of the votes cast with respect to that director’s election at any meeting for the election of directors at which a quorum is present, except in the case of contested elections, as determined in accordance with our Bylaws, in which case directors shall be elected by a plurality of the votes cast.

Voting Rights. Each holder of Common Stock is entitled to one vote for each share held of record on all matters to be voted upon by stockholders.

Dividends. Subject to the rights, powers and preferences of any outstanding Preferred Stock, and except as provided by law or in our Certificate of Incorporation, dividends may be declared and paid or set aside for payment on the Common Stock out of legally available assets or funds when and as declared by the board of directors.

Liquidation, Dissolution and Winding Up. Subject to the rights, powers and preferences of any outstanding Preferred Stock, in the event of our liquidation, dissolution or winding up, our net assets will be distributed pro rata to the holders of our Common Stock.

Other Rights. Holders of the Common Stock have no right to:

•convert the stock into any other security;

•have the stock redeemed;

•purchase additional stock; or

•maintain their proportionate ownership interest.

The Common Stock does not have cumulative voting rights. Holders of shares of the Common Stock are not required to make additional capital contributions.

Board of Directors. All of our directors are elected annually. The number of directors comprising our board of directors is fixed from time to time by the board of directors.

Stockholder Nomination of Directors. Our Bylaws provide that a stockholder must notify us in writing of any stockholder nomination of a director not earlier than the close of business on the 120th day, and not later than the close of business on the 90th day, prior to the first anniversary of the preceding year’s annual meeting; provided, that if the date of the annual meeting is more than 30 days before or more than 70 days after such anniversary date, notice by the stockholder to be timely must be so delivered not earlier than the close of business on the 120th day prior to the date of such annual meeting and not later than the close of business on the 10th day following the day on which public announcement of the date of such annual meeting is first made by us. Our Bylaws also provide that, subject to certain limitations, if a stockholder (or a qualified representative of the stockholder) does not appear at a meeting of stockholders to present a nomination, such nomination shall be disregarded, notwithstanding that proxies in respect of such vote may have been received by us.

No Action by Written Consent. Our Certificate of Incorporation provides that our stockholders may not act by written consent and may only act at duly called meetings of stockholders.

Delaware Business Combination Statute. Section 203 of the General Corporation Law of the State of Delaware, which we refer to as the DGCL, is applicable to us. Section 203 of the DGCL restricts some types of transactions and business combinations between a corporation and a 15% stockholder. A 15% stockholder is generally considered by Section 203 to be a person owning 15% or more of the corporation’s outstanding voting stock. Section 203 refers to a 15% stockholder as an “interested stockholder.” Section 203 restricts these transactions for a period of three years from the date the stockholder acquires 15% or more of our outstanding voting stock. With some exceptions, unless the transaction is approved by the board of directors and the holders of at least two-thirds of the outstanding voting stock of the corporation, Section 203 prohibits significant business transactions such as:

•a merger with, disposition of significant assets to or receipt of disproportionate financial benefits by the interested stockholder, and

•any other transaction that would increase the interested stockholder’s proportionate ownership of any class or series of our capital stock.

The shares held by the interested stockholder are not counted as outstanding when calculating the two-thirds of the outstanding voting stock needed for approval.

The prohibition against these transactions does not apply if:

•prior to the time that any stockholder became an interested stockholder, the board of directors approved either the business combination or the transaction in which such stockholder acquired 15% or more of our outstanding voting stock, or

•the interested stockholder owns at least 85% of our outstanding voting stock as a result of a transaction in which such stockholder acquired 15% or more of our outstanding voting stock. Shares held by persons who are both directors and officers or by some types of employee stock plans are not counted as outstanding when making this calculation.

Effects of Authorized but Unissued Stock. We have shares of Common Stock and Preferred Stock available for future issuance without stockholder approval, subject to any limitations imposed by the listing standards of The Nasdaq Global Select Market. We may utilize these additional shares for a variety of corporate purposes, including for future public offerings to raise additional capital or facilitate corporate acquisitions or for payment as a dividend on our capital stock. The existence of unissued and unreserved Common Stock and Preferred Stock may enable our board of directors to issue shares to persons friendly to current management or to issue Preferred Stock with terms that could have the effect of making it more difficult for a third party to acquire, or could discourage a third party from seeking to acquire, a controlling interest in our company by means of a merger, tender offer, proxy contest or otherwise. In addition, if we issue Preferred Stock, the issuance could adversely affect the voting power of holders of Common Stock.

Our Certificate of Incorporation provides that a member of the board of directors will not be personally liable to us or our stockholders for monetary damages for breaches of their legal duties to us or our stockholders as a director, except for liability:

•for any breach of the director’s duty of loyalty to us or our stockholders;

•for acts or omissions by the director not in good faith or which involve intentional misconduct or a knowing violation of the law;

•for declaring dividends or authorizing the purchase or redemption of shares in violation of Delaware law; or

•for transactions where the director derived any improper personal benefit.

Our Certificate of Incorporation also allows us to indemnify directors and officers to the fullest extent authorized by Delaware law.

Our Bylaws provide that we shall, to the fullest extent authorized by the DGCL, indemnify our directors; provided, however, that we may limit the extent of such indemnification by individual contracts with our directors; and, provided, further, that we shall not be required to indemnify any director in connection with any proceeding (or part thereof) initiated by such person or any proceeding by such person against us or our directors, officers, employees or other agents unless (i) such indemnification is expressly required to be made by law, (ii) the proceeding was authorized by our board of directors, or (iii) such indemnification is provided by us, in our board’s sole discretion, pursuant to our powers under the DGCL.