DMTX Dimension Therapeutics, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO

TENDER OFFER STATEMENT UNDER SECTION 14(D)(1)

OR 13(E)(1) OF THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 6)

DIMENSION THERAPEUTICS, INC.

(Name of Subject Company (Issuer))

MYSTIC RIVER MERGER SUB INC.

a wholly-owned subsidiary of

ULTRAGENYX PHARMACEUTICAL INC.

(Names of Filing Persons (Offeror))

Common Stock, Par Value $0.0001 Per Share

(Title of Class of Securities)

25433V105

(Cusip Number of Class of Securities)

Karah Parschauer

Executive Vice President and General Counsel

Ultragenyx Pharmaceutical Inc.

60 Leveroni Court

Novato, California 94949

415.483.8800

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications on Behalf of Filing Persons)

Copies to:

Graham Robinson

Laura Knoll

Skadden, Arps, Slate, Meagher & Flom LLP

500 Boylston Street

Boston, MA 02210

(617) 573-4850

CALCULATION OF FILING FEE

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer.  ☒

This Amendment No. 6 (this “Amendment”) amends and supplements the Tender Offer Statement on Schedule TO filed by Mystic River Merger Sub Inc., a Delaware corporation (“Purchaser”), and Ultragenyx Pharmaceutical Inc., a Delaware corporation (“Parent”), with the U.S. Securities and Exchange Commission on October 10, 2017 (together with any subsequent amendments and supplements thereto, the “Schedule TO”). The Schedule TO relates to the offer by Purchaser to purchase all outstanding shares of common stock, $0.0001 par value per share (“Shares”), of Dimension Therapeutics, Inc., a Delaware corporation (“Dimension”), at a price of $6.00 per Share, net to the holder in cash, without interest upon the terms and subject to the conditions described in the Offer to Purchase dated October 10, 2017 (together with any amendments or supplements thereto, the “Offer to Purchase”) and in the accompanying Letter of Transmittal (together with any amendments or supplements thereto, the “Letter of Transmittal,” and with the Offer to Purchase, the “Offer”), which are annexed to and filed with this Schedule TO as Exhibits (a)(1)(A) and (a)(1)(B), respectively. Purchaser is a wholly-owned subsidiary of Parent. This Amendment is being filed on behalf of Parent and Purchaser. Unless otherwise indicated, references to sections in this Schedule TO are references to sections of the Offer to Purchase.

Amendments to the Offer to Purchase

Items 1 through 11.

The information set forth in the Offer to Purchase and Items 1 through 11 of the Schedule TO, to the extent such Items incorporate by reference the information contained in the Offer to Purchase, are hereby amended and supplemented as follows:

“At 12:00 midnight, Eastern time, on Tuesday, November 7, 2017 (one minute after 11:59 p.m., Eastern time, on November 6, 2017), the Offer expired. The Depositary advised Purchaser that, as of the expiration of the Offer, a total of 21,947,871 Shares were validly tendered and not properly withdrawn pursuant to the Offer, representing approximately 86.5% of the Shares outstanding as of the expiration of the Offer. The Depositary advised Purchaser that Notices of Guaranteed Delivery have been delivered with respect to 125,702 Shares, representing less than one percent of the outstanding Shares as of the expiration of the Offer.

As of the expiration of the Offer, the number of Shares validly tendered and not properly withdrawn pursuant to the Offer satisfied the Minimum Tender Condition, and all other conditions to the Offer were satisfied or waived. Immediately after the expiration of the Offer, Purchaser irrevocably accepted for payment, and will promptly pay for, all Shares tendered and not properly withdrawn pursuant to the Offer.

Following expiration of the Offer and acceptance for payment of the Shares, Parent and Purchaser had ownership sufficient to effect the Merger under Section 251(h) of the DGCL without a vote of the Dimension stockholders. Accordingly, Purchaser has effected the Merger in which Purchaser merged with and into Dimension with Dimension surviving the Merger and continuing as a wholly-owned subsidiary of Parent. At the effective time of the Merger, each issued and outstanding Share other than Shares owned by Parent, Purchaser or Dimension (other than Shares held in trust accounts, managed accounts and the like, or otherwise held in a fiduciary or agency capacity, that are beneficially owned by third parties or Shares for which a Dimension stockholder has properly exercised appraisal rights under Delaware law) was canceled and converted into the right to receive $6.00, in cash, without interest.

Following consummation of the Merger, the Shares ceased to trade on NASDAQ prior to the opening of business on November 7, 2017, and Dimension has requested that NASDAQ file a Notification of Removal from Listing and/or Registration under Section 12(b) of the Exchange Act on Form 25 to delist and deregister the Shares. Parent and Purchaser intend to take steps to cause the termination of the registration of the Shares under the Exchange Act and suspend all of Dimension’s reporting obligations under the Exchange Act as promptly as practicable.

On November 7, 2017, Parent issued a press release announcing the expiration and results of the Offer. The full text of the press release is attached as Exhibit (a)(5)(H) hereto, and is incorporated herein by reference.”

Item 12.

Item 12 of the Schedule TO is hereby amended and supplemented by adding the following exhibit:

SIGNATURE

After due inquiry and to the best of my knowledge and belief, the undersigned certify that the information set forth in this statement is true, complete and correct.

Dated: November 7, 2017

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