Exhibit 10.1

May 19, 2016

John Cox
559 Concord Road
Sudbury, MA 01776

Re: Conditional Offer of SpinCo Employment

Dear John,

This letter agreement (“Agreement”) sets forth the terms and conditions of our
conditional offer of employment with the new legal entity expected to be spun
out from Biogen Inc. to focus on the research, development, and
commercialization of hemophilia therapies. For purposes of this letter
agreement, the new legal entity will be referred to as “SpinCo” and the date of
the separation and distribution of SpinCo shares to SpinCo shareholders (i.e.,
the date the spin out occurs) will be referred to as the “Date of Distribution.”
Provided SpinCo has been legally formed, the spin-off occurs, and you have met
all of the terms and conditions for employment with SpinCo as set forth below,
if you accept this offer your new full-time position with SpinCo will be as its
Chief Executive Officer.

1.
Compensation and Benefits:

If the contingencies set forth below are met, and you become employed by SpinCo
under this Agreement, your employment shall be at-will, and your compensation
and benefits package will be as follows:

The SpinCo Offer

Salary: This is a full-time, exempt position and your starting annual salary at
SpinCo will be $800,000.00, and will be paid in accordance with SpinCo’s
standard payroll practices and policies in effect from time to time.

Annual Cash Bonus: You will be eligible to receive an annual cash target bonus
equal to 100% of your annual base salary. Based upon your start date, your
annual target bonus amount for the 2016 performance year may be pro-rated for
the time served as EVP, PO&T for Biogen and the time spent as the CEO under the
SpinCo bonus plan based on the compensation that was effective at such time. At
this time final details with respect to the SpinCo annual bonus plan have not
been finalized. The annual cash bonus shall be payable in a single lump sum as
soon as reasonably practicable following the close of the year with respect to
which the bonus is paid, but in all events prior to March 15 of the year
following the year with respect to which the bonus is paid.

Founders’ Stock Option Grant: In connection with the commencement of your
employment with SpinCo, on or near your commencement date you will be provided a
one-time grant of SpinCo stock options with an exercise price equal to the fair
market value of SpinCo stock on the date of grant (“Founders’ Grant”). The
approximate grant date expected value of your SpinCo stock options will be
$6,500,000.00. These stock options will be subject to a three-year cliff vesting
(i.e., they will all vest three years after the date of grant) and will be
granted on or shortly after the Date of Distribution, following your start date.

The actual terms of your Founders’ Grant stock options will be communicated to
you following the grant date. Your Founders’ Grant will be awarded under a
SpinCo Omnibus Equity Plan, which plan has not yet been finalized.

Each year, typically in the first quarter of the year, you will be eligible to
receive a SpinCo annual Long Term Incentive (LTI) award, which will be based on
the planning range in effect at the time of grant. The current estimated
planning reference point is $6,500,000.00 and the delivery vehicle is expected
to be 50% SpinCo restricted stock units and 50% SpinCo stock options. Final
determination of the SpinCo annual LTI award amounts and delivery vehicles will
be made by the SpinCo compensation committee of the Board of Directors of SpinCo
once it is formed.

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Enhanced Severance Benefit: If, (1) during the time period beginning with the
date of this letter through the Date of Distribution, you are involuntarily
terminated by Biogen other than “For Cause” (as defined in Biogen’s Amended and
Restated 2008 Omnibus Equity Plan)  or (2) on or after the Date of Distribution
through the first anniversary of the Date of Distribution, you are involuntarily
terminated by SpinCo other than “For Cause” (as defined in Biogen’s Amended and
Restated 2008 Omnibus Equity Plan)  or you experience an “Involuntary Employment
Action” following a “Corporate Change in Control”  (each as defined in Biogen’s
Amended and Restated 2008 Omnibus Equity Plan) , you will be entitled to receive
(i) a lump sum severance payment equal to 24 months of salary and target bonus,
(ii) up to 12 months of executive level outplacement services from a recognized
provider of such services for C-level executives selected by the employer
company, and (iii) continued subsidized health benefits for 21 months provided
you timely complete and submit your COBRA election form and continue to pay
timely the employee portion of the premiums. In addition, payment and provision
of these severance benefits are conditioned upon your signing an irrevocable
general release in favor of the employer company, in form and substance
acceptable to the employer company, with respect to any and all claims relating
to your employment and the termination of your employment with the employer
company.

In addition, if between July 1, 2016 and the expected Date of Distribution
(expected to be Q4 2016 or Q1 2017) (1) an acquirer is identified for the
hemophilia business and/or the SpinCo spin out is not expected to occur or does
not occur, and (2) your current Biogen position is no longer available for you
to return to, then, in addition to the benefits described in the above
paragraph, you shall be entitled to acceleration of all (100%) of your
outstanding Biogen LTI awards that are unvested as of your termination date in
accordance with the terms of the award agreements, including the requirements of
Section 409A of the Internal Revenue Code.

The enhanced severance benefits described in the above paragraphs will be
provided to you in lieu of and not in addition to any severance benefits you
would have been eligible for under the Severance Plan for U.S. Executive Vice
Presidents effective January 1, 2014; provided, however, that if, after the
first anniversary of the Date of Distribution, your Spinco employment
terminates, then you will be eligible for severance benefits under the SpinCo
severance plan for executives in effect at the time of termination.

Change in Control: It is expected that the Spinco omnibus equity plan and
executive severance plan will contain standard change in control provisions
similar to those contained in the comparable Biogen plans.
Benefits: You may participate in all SpinCo qualified employee benefit plans,
welfare benefit plans and programs for which you are eligible, in accordance
with the terms and conditions of such plans and programs as in effect and as
they may be amended from time to time. Your participation in some or all of
these plans may be contingent upon your execution of, and the acceptance by the
plan’s administrator of, a participation agreement, and upon your satisfaction
of other terms and conditions. Except as specifically set forth herein, the
benefits provided under this Agreement shall in no way alter or affect the terms
and conditions of any SpinCo sponsored or maintained qualified employee benefit
plans, non-qualified employee benefits plans, programs, and welfare benefit
plans in which you may otherwise be eligible to participate, and your
eligibility to participate in any such plans or programs will be determined in
accordance with the terms and conditions of such plans or programs, as they may
be amended from time to time.

At this time, the compensation and benefit plans, programs and arrangements to
be offered to employees of SpinCo have not been finalized. However, SpinCo will
offer its employees market competitive benefits plans (and comparable to
Biogen’s plans), with the 401k and the non-qualified retirement plans generally
comparable to the current Biogen plans. All such benefit plans will be in place
by your start date.

2.
Other Matters:

  
Share Ownership Requirement: It will be important for SpinCo to ensure strong
alignment between the interests of its senior executives and those of SpinCo
stockholders. It is expected that through your SpinCo annual long-term incentive
grants, you will accumulate and retain SpinCo shares in an amount equivalent to
a specified multiple of your salary within a reasonable period of time that is
yet to be determined.

Signed Proprietary Agreement: As a condition of employment, you will be required
to sign SpinCo’s form of “Employee Proprietary Information and Inventions and
Dispute Resolution Agreement”’ as a condition of employment. This agreement has
not been finalized but it is expected to be substantially similar to the Biogen
agreement currently used with its employees.

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Treatment of Unvested Biogen LTI Awards: The unvested and outstanding Biogen LTI
Awards will be converted to SpinCo shares as follows: (a) Restricted Stock Units
(RSUs) - all unvested Biogen RSUs will convert into an equivalent value of
SpinCo RSUs at or near the Date of Distribution and continue their regular
vesting schedule; (b) Market Stock Units (MSUs) - at or near Date of
Distribution we will determine the performance multiplier based on the then
30-day average of Biogen closing stock price and derive the number of Biogen
shares earned; we will convert the shares earned to an equivalent value of
SpinCo RSUs and continue their regular vesting schedule and (c) Cash Settled
Performance Units (CSPUs) - at or near the Date of Distribution we will apply a
projected performance multiplier to calculate the shares earned, then use the
30-day average of Biogen’s closing stock price at or near the Date of
Distribution to derive the CSPU value. This value will be converted into an
equivalent value of SpinCo RSUs and continue their regular vesting schedule.

Indemnification: You will be indemnified pursuant to an indemnification
agreement that you will enter into with SpinCo that is expected to contain
provisions similar to the indemnification agreement you currently have with
Biogen.

3.
Contingencies:

This SpinCo offer of employment is contingent upon: (1) the actual legal
formation of SpinCo; (2) the spin-off occurring on terms approved by the board
of directors of Biogen Inc.; (3) your formal acceptance of this offer within the
time frame set forth below; and (4) you remaining employed by Biogen, continuing
to perform your central duties of transitioning your current role to a successor
and assisting with the establishment of SpinCo, until you become employed by
SpinCo. The Distribution Date (i.e., the date on which SpinCo is spun off from
Biogen Inc.) shall be your date of hire with SpinCo unless you are notified
otherwise in writing (“SpinCo Hire Date”). Until the SpinCo Hire Date, the terms
and conditions of your current at-will Biogen employment will remain in effect
(except as noted in the “Enhanced Severance Benefit” section), although you will
be expected to work on matters associated with SpinCo in the interim.

Following your SpinCo Hire Date, as a condition of maintaining your employment
with SpinCo, you agree that you must: comply with any and all SpinCo hiring and
employment policies, practices and procedures, including but not limited to, the
execution of any forms provided by SpinCo in connection with your employment and
if required, presenting suitable documentation for I-9 INS certification.

4.
Additional Terms:

(a)
At-Will Employment. Your employment with SpinCo will be at-will at all times, as
it is currently with Biogen. This means that both you and SpinCo shall always
have the right to terminate your employment with SpinCo at any time for any
reason, with or without notice or cause. Nothing in this Agreement shall be
construed as a contract or guarantee of continued employment with either Biogen
or SpinCo for any length of time.

(b)
Employment Files. In connection with your employment at SpinCo, you agree to,
and hereby authorize the disclosure and transfer of copies or originals of all
of your personnel, medical, benefits and other employment-related files and
information from Biogen to SpinCo upon your SpinCo Hire Date. Among other
things, such transfer of your files or information may be necessary to ensure
you receive appropriate service credits or meet other benefit eligibility
requirements. You agree to execute any lawful and reasonable authorizations
requested by SpinCo or Biogen to effectuate such transfers.

(c)
Integration; Amendment. This Agreement supersedes any and all prior oral and/or
written agreements or understandings with respect to your contemplated
employment by SpinCo and sets forth the entire agreement between Biogen and you
with respect to your contemplated employment by SpinCo. No provision of this
Agreement may be modified, amended or waived except in a writing signed by both
parties.

(d)
Applicable Law. The validity, interpretation and performance of this Agreement,
shall be governed by, and construed in accordance with, the internal laws of
Massachusetts, without giving effect to conflict of law principles.

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(e)
Severability and Execution. The provisions of this Agreement are severable, and
if for any reason any part hereof shall be found to be unenforceable, the
remaining provisions shall be enforced in full. This Agreement may be executed
in counterparts, each of which shall be an original, but all of which together
shall constitute one agreement. Execution of a facsimile copy of this Agreement
shall have the same force and effect as execution of an original, and a
facsimile signature shall be deemed an original and valid signature.

(f)
Acknowledgement. By signing the Agreement in the space provided below, you are
acknowledging that you have read and understand all of the terms of this
Agreement and are entering into this Agreement voluntarily.

(g)
Rule 409A and Rule 280G. Please see Exhibit A.

If you have any questions, please feel free to contact me.

Best regards,

/s/ Robert Pangia
Robert Pangia
Compensation and Management Development Committee, Chairman
Biogen Inc. Board of Directors

I accept this conditional offer of employment and acknowledge the contingencies
of employment described above, including the at-will nature of my employment.

ACCEPTED:

/s/ John Cox                    John Cox            May 19, 2016
Signature     Name (Print) Date

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Exhibit A

Rule 409A: The parties intend that any amounts payable hereunder comply with or
are exempt from Section 409A of the Internal Revenue Code of 1986, as amended
(“Section 409A”), and this Agreement shall be administered accordingly.
Notwithstanding the foregoing, the Company does not guarantee any particular tax
effect, and you shall be solely responsible and liable for the satisfaction of
all taxes, penalties and interest that may be imposed on you or for your account
in connection with the Agreement (including any taxes, penalties and interest
under Section 409A), and neither the Company nor any of its affiliates shall
have any obligation to indemnify or otherwise hold you (or any beneficiary)
harmless from any or all of such taxes, penalties or interest. 

For purposes of Section 409A, each of the payments that may be made under this
Agreement shall be deemed to be a separate payment for purposes of Section 409A
and the right, if any, to a series of installment payments under this Agreement
is to be treated as a right to a series of separate payments.

With respect to the time of payments of any amounts under the Agreement that are
“deferred compensation” subject to Section 409A, references in the Agreement to
“termination of employment” (and substantially similar phrases) shall mean
“separation from service” within the meaning of Section 409A. (after giving
effect to the presumptions contained therein). Notwithstanding anything to the
contrary in this Agreement, if at the time of your termination of employment,
you are a “specified employee,” as defined below, any and all amounts payable
under this Agreement on account of such separation from service that constitute
deferred compensation and would (but for this provision) be payable within six
(6) months following the date of termination, shall instead be paid on the next
business day following the expiration of such six (6) month period or, if
earlier, upon your death; except (A) to the extent of amounts that do not
constitute a deferral of compensation within the meaning of Treasury regulation
Section 1.409A-1(b) (including without limitation by reason of the safe harbor
set forth in Section 1.409A-1(b)(9)(iii), as determined by the Company in its
reasonable good faith discretion); (B) benefits that qualify as excepted welfare
benefits pursuant to Treasury regulation Section 1.409A-1(a)(5); or (C) other
amounts or benefits that are not subject to the requirements of Section 409A.
The term “specified employee” means an individual determined by the Company to
be a specified employee under Treasury regulation Section 1.409A-1(i).
For the avoidance of doubt, it is intended that any expense reimbursement made
to you hereunder shall be exempt from Section 409A. Notwithstanding the
foregoing, if any expense reimbursement made hereunder shall be determined to be
“deferred compensation” within the meaning of Section 409A, then (i) the amount
of the indemnification payment or expense reimbursement during one taxable year
shall not affect the amount of the expense reimbursement during any other
taxable year, (ii) the expense reimbursement shall be made on or before the last
day of your taxable year following the year in which the expense was incurred
and (iii) the right to expense reimbursement hereunder shall not be subject to
liquidation or exchange for another benefit.

Section 280G: Notwithstanding anything to the contrary contained in the this
Agreement or any other agreement or arrangement between you and SpinCo, to the
extent that any of the payments and benefits provided for under this Agreement
or such other agreement or arrangement (collectively, the “Payments”) would
constitute an “excess parachute payment” within the meaning of section 280G of
the Internal Revenue Code of 1986, as amended (the “Code”), the amount of such
Payments shall be reduced to the amount that would result in no portion of the
Payments being subject to the excise tax imposed pursuant to section 4999 of the
Code.