UNITED STATES
                  SECURITIES AND EXCHANGE COMMISSION
                        Washington, D.C. 20549


                             SCHEDULE 13G

               Under the Securities Exchange Act of 1934


                          (Amendment No. 1)*


                        Rubius Therapeutics, Inc.
                             (Name of Issuer)

                               Common Stock
                      (Title of Class of Securities)

                                78116T103
                              (CUSIP Number)

                            December 31, 2019
         (Date of Event Which Requires Filing of this Statement)


Check the appropriate box to designate the rule pursuant to which this
Schedule is filed:
[X] Rule 13d-1(b)
[ ] Rule 13d-1(c)
[ ] Rule 13d-1(d)


*The remainder of this cover page shall be filled out for a reporting
person's initial filing on this form with respect to the subject class
of securities, and for any subsequent amendment containing information
which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not
be deemed to be "filed" for the purpose of Section 18 of the Securities
Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of
that section of the Act but shall be subject to all other provisions of
the Act (however, see the Notes).


















CUSIP: 78116T103                                                Page 1 of 5

1 NAMES OF REPORTING PERSONS I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY) Capital World Investors ** 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a) (b) 3 SEC USE ONLY 4 CITIZENSHIP OR PLACE OF ORGANIZATION Delaware 5 SOLE VOTING POWER 5,177,000 6 SHARED VOTING POWER NUMBER OF SHARES NONE BENEFICIALL Y OWNED BY 7 SOLE DISPOSITIVE POWER EACH REPORTING 5,177,000 PERSON WITH: 8 SHARED DISPOSITIVE POWER NONE 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 5,177,000 Beneficial ownership disclaimed pursuant to Rule 13d-4 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 6.4% 12 TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IA ** A division of Capital Research and Management Company (CRMC) CUSIP: 78116T103 Page 2 of 5

SECURITIES AND EXCHANGE COMMISSION Washington, DC 20549 Schedule 13G Under the Securities Exchange Act of 1934 Amendment No. 1 Item 1(a) Name of Issuer: Rubius Therapeutics, Inc. Item 1(b) Address of Issuer's Principal Executive Offices: 399 BINNEY STREET, SUITE 300 CAMBRIDGE MA 02139 Item 2(a) Name of Person(s) Filing: Capital World Investors Item 2(b) Address of Principal Business Office or, if none, Residence: 333 South Hope Street Los Angeles, CA 90071 Item 2(c) Citizenship: N/A Item 2(d) Title of Class of Securities: Common Stock Item 2(e) CUSIP Number: 78116T103 Item 3 If this statement is filed pursuant to sections 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a: (e) [X] An investment adviser in accordance with section 240.13d-1(b)(1)(ii)(E). Item 4 Ownership Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1. (a) Amount beneficially owned: (b) Percent of class: (c) Number of shares as to which the person has: (i) Sole power to vote or to direct the vote: (ii) Shared power to vote or to direct the vote: (iii) Sole power to dispose or to direct the disposition of: (iv) Shared power to dispose or to direct the disposition of: See page 2 Capital World Investors divisions of CRMC and Capital International Limited collectively provide investment management services under the name Capital World Investors. Capital World Investors is deemed to be the beneficial owner of 5,177,000 shares or 6.4% of the 79,800,858 shares believed to be outstanding. CUSIP: 78116T103 Page 3 of 5

Item 5 Ownership of Five Percent or Less of a Class. If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following: [ ] Item 6 Ownership of More than Five Percent on Behalf of Another Person: One or more clients of Capital World Investors have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the Common Stock of Rubius Therapeutics, Inc.. Capital World Investors holds more than five percent of the outstanding Common Stock of Rubius Therapeutics, Inc. as of December 31, 2019 on behalf of each of the following client(s): SMALLCAP World Fund, Inc. Item 7 Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person: N/A Item 8 Identification and Classification of Members of the Group: N/A Item 9 Notice of Dissolution of Group: N/A Item 10 Certification By signing below, I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect. Signature After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Date: February 12, 2020 Signature: /s/ Michael J. Triessl Name/Title: Michael J. Triessl - Senior Vice President and Senior Counsel, Fund Business Management Group Capital Research and Management Company CUSIP: 78116T103 Page 4 of 5 CUSIP: 78116T103 Page 5 of 5