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                               May 7, 2020

       Geoff McDonough, M.D.
       President and Chief Executive Officer
       Generation Bio Co.
       301 Binney Street
       Cambridge, MA 02142

                                                        Re: Generation Bio Co.
                                                            Draft Registration
Statement on Form S-1
                                                            Submitted April 10,
2020
                                                            CIK No. 0001733294

       Dear Dr. McDonough:

              We have reviewed your draft registration statement and have the
following comments. In
       some of our comments, we may ask you to provide us with information so
we may better
       understand your disclosure.

              Please respond to this letter by providing the requested
information and either submitting
       an amended draft registration statement or publicly filing your
registration statement on
       EDGAR. If you do not believe our comments apply to your facts and
circumstances or do not
       believe an amendment is appropriate, please tell us why in your
response.

             After reviewing the information you provide in response to these
comments and your
       amended draft registration statement or filed registration statement, we
may have additional
       comments.

       Draft Registration Statement on Form S-1

       Overview, page 1

   1.                                                   Please clarify here in
the Overview, and throughout your registration statement where
                                                        appropriate, that you
are a preclinical stage company and that you currently have no
                                                        marketable products or
product candidates. In this regard, we note your disclosure on
                                                        page 5 and page 16
regarding risks associated with your business and the status of your
                                                        research and
development.
   2.                                                   Please revise to
clarify the term "capsid-free" and briefly explain its significance to your
                                                        manufacturing process.
 Geoff McDonough, M.D.
FirstName LastNameGeoff McDonough, M.D.
Generation Bio Co.
Comapany NameGeneration Bio Co.
May 7, 2020
Page 2
May 7, 2020 Page 2
FirstName LastName
Our portfolio, page 3

3.       Please clarify what "lead optimization" means as it relates to your
graphic here and at
         page 110. Also, for context, revise your graphic to indicate the
material stages you will
         need to complete before marketing your product. For instance, include
columns for each
         of Phase 1, 2 and 3 clinical trials.
4.       Please revise the pipeline chart to identify the programs listed as
"undisclosed" and
         specify the indications they target, or remove these programs from the
chart here and in
         the Business section.
5.       Briefly expand your Summary to disclose the status of intellectual
property that is material
         to your portfolio. For example, state whether you own the intellectual
property, whether
         you have any patent applications, or if you license intellectual
property. Also indicate any
         license upon which your research and development depends.
Use of Proceeds, page 85

6.       We note your disclosure that one intended use of the proceeds from
this offering is to fund
         continued research and development of your programs, including
preclinical and IND-
         enabling studies. Please specify the programs that will be advanced
and how far into the
         preclinical or IND-enabling studies you expect to reach with the
proceeds from this
         offering.
Business
Summary of limitations of current approaches, page 114

7.       In your graphic and throughout your disclosure where you discuss the
drawbacks of
         therapy approaches other than your own, clarify that your approach has
not yet been
         approved for treatment and that the contrasts with other approaches
may not be direct
         comparison. Please also balance your disclosure, as applicable, to
discuss whether there
         are challenges or uncertainties with respect to developing your
therapies.
Our non-viral gene therapy platform, page 117

8.       Please revise so that the charts and graphics in this section are
legible and readers are able
         to tell what the graphic is intended to describe. For instance, please
enlarge the graphs on
         the right on page 117 so that the axes and legends are legible. On
page 118, please
         include a narrative describing what the graphic under "ceDNA purity at
200-liter
         manufacturing scale" is intended to depict.
License agreements
License agreement with the National Institute of Health, page 148

9.       Please disclose when the last-to-expire patent is scheduled to expire.
Provide similar
         disclosure with respect to the University of Massachusetts license
agreement.
 Geoff McDonough, M.D.
Generation Bio Co.
May 7, 2020
Page 3
Executive compensation
Summary compensation table, page 184

10.      We note that in your narrative you state that none of your executive
officers named
         executive officers is currently party to an employment agreement or
other agreement or
         arrangement that provides for automatic or scheduled increases in base
salary. Please
         clarify and disclose where you deem appropriate whether you have any
material
         employment agreements covering any other aspect of employment. If so,
please file any
         such agreements as exhibits to your registration statement.
Description of capital stock
Preferred stock, page 205

11.      Provide the conversion ratio for each class of preferred shares that
will automatically
         convert into common stock upon the closing of this offering.
Registration rights, page 206

12.      Expand your disclosure to discuss the circumstances under which the
referenced holder
         will have the right to require you to register their shares under the
Securities Act.
Description of Capital Stock
Exclusive Forum Provision, page 208

13.      We note your disclosure that the exclusive forum provision does not
apply to suits brought
         to enforce a duty or liability created by the Securities Act, the
Exchange Act or any other
         claim for which federal courts have exclusive jurisdiction. Your form
of restated
         certificate of incorporation, filed as Exhibit 3.1, does not reference
either Act. Please
         revise the forum selection provision for consistency with disclosure
in the prospectus. In
         addition, please revise your disclosure to state that investors will
not be deemed to have
         waived the company's compliance with the federal securities laws and
the rules and
         regulations thereunder.
General

14.    Please supplementally provide us with copies of all written
communications, as defined in
FirstName LastNameGeoff McDonough, M.D.
       Rule 405 under the Securities Act, that you, or anyone authorized to do
so on your behalf,
Comapany NameGeneration Bio Co.in reliance on Section 5(d) of the Securities
Act, whether or
       present to potential investors
May 7,not they retain copies of the communications.
        2020 Page 3
FirstName LastName
 Geoff McDonough, M.D.
FirstName LastNameGeoff McDonough, M.D.
Generation Bio Co.
Comapany NameGeneration Bio Co.
May 7, 2020
Page 4
May 7, 2020 Page 4
FirstName LastName
       You may contact Gary Newberry at 202-551-3761 or Lynn Dicker at
202-551-3616 if
you have questions regarding comments on the financial statements and related
matters. Please
contact Courtney Lindsay at 202-551-7237 or Mary Beth Breslin at 202-551-3625
with any other
questions.



                                                         Sincerely,

                                                         Division of
Corporation Finance
                                                         Office of Life
Sciences
cc:      Molly Fox