As filed with the Securities and Exchange Commission on November 5, 2013

Registration No. 333-        

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM S-1
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

KARYOPHARM THERAPEUTICS INC.
(Exact Name of Registrant as Specified in its Charter)

2 Mercer Road
Natick, MA 01760
(508) 975-4820
(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant's Principal Executive Offices)

Michael G. Kauffman, M.D., Ph.D.
Chief Executive Officer
Karyopharm Therapeutics Inc.
2 Mercer Road
Natick, MA 01760
(508) 975-4820
(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Approximate date of commencement of proposed sale to the public:
As soon as practicable after this Registration Statement becomes effective.

         If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.    o

         If this Form is filed to register additional securities for an offering pursuant Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ý 333-191584

         If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

         If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

         Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of "large accelerated filer," "accelerated filer" and "smaller reporting company" in Rule 12b-2 of the Exchange Act. (Check one):

CALCULATION OF REGISTRATION FEE

(1)

Includes 170,000 shares which the Underwriters have the option to purchase from the Company to cover over-allotments, if any.

(2)

Estimated in accordance with Rule 457(a) of the Securities Act of 1933 solely on the basis of $16.00, the Price to the Public set forth on the cover page of the Registrant's Prospectus dated November 5, 2013 relating to its initial public offering pursuant to the Company's Registration Statement on Form S-1 (File No. 333-191584).

EXPLANATORY NOTE AND INCORPORATION BY REFERENCE

        This registration statement is being filed with respect to the registration of additional shares of common stock, par value $0.0001 per share, of Karyopharm Therapeutics Inc., a Delaware corporation, pursuant to Rule 462(b) under the Securities Act of 1933, as amended. The contents of the earlier registration statement on Form S-1(File No. 333-191584), which was declared effective by the Commission on November 5, 2013, are incorporated in this registration statement by reference.

        The required opinions and consents are listed on an Exhibit Index attached hereto and filed herewith.

SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, the Registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Natick, Commonwealth of Massachusetts, on this 5^th day of November, 2013.

        Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

EXHIBIT INDEX

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