Exhibit 10.1

Altus Pharmaceuticals, Inc.

May 21, 2008

Georges Gemayel, Ph.D.
[Address]

Dear Georges:

We are pleased to extend to you an offer to join Altus Pharmaceuticals Inc.
(“Altus” or the “Company”) as President and Chief Executive Officer and a member
of the Board of Directors. Through the discussions the Board of Directors has
had with you during the interview process, we have been impressed with your
substantial pharmaceutical industry experience, track record of accomplishments,
strong business intellect, and overall leadership skills. Our impressions have
been reinforced by the extremely positive set of reference calls that we also
conducted. We believe you are well qualified to assume the Chief Executive
Officer role at Altus and have every confidence that you will be successful in
this capacity.

Commensurate with this position, the Compensation Committee of the Board of
Directors offers you the following exceptional compensation package. The
position provides an initial annualized base salary of $540,000 which will be
paid on a biweekly basis. Annually beginning in 2009, you will be eligible for
salary increases at the discretion of the Compensation Committee. In addition to
your base salary, you will have an opportunity to earn an initial target annual
performance bonus of 50% of your earned salary based on achievement of a series
of personal and Company objectives that the Board of Directors acting through
the Compensation Committee and you will define annually. For 2008, objectives
will be recommended by you for the Board’s review and approval thirty days after
the Commencement Date (as defined below). The 2008 performance bonus will be
awarded based on achievement against these objectives and prorated based on the
Commencement Date. As part of the Compensation Committee’s annual executive
compensation review process, the Committee will review the target bonus level
and, at its discretion, may increase the target bonus opportunity consistent
with market practices and Altus’ compensation philosophy.

The President and Chief Executive Officer position will provide you with an
initial grant of stock options exercisable for a total of 900,000 shares of
Altus common stock at an exercise price on the closing price of our common stock
price as of your first day of employment. Your stock options will vest over a
four-year schedule so long as you remain an employee. The first twenty-five
percent of your shares will vest on your anniversary date, and the remaining
seventy-five percent will vest quarterly over the remaining three years. Within
12 months following a Change in Control, in the event you are terminated without
Cause or you resign for Good Reason, your unvested options will vest in full in
accordance with our executive stock option agreements. In the event you are
terminated without Cause or you resign for Good Reason in other circumstances,
all, a portion or none of the unvested options may be accelerated to the date of
termination at the discretion of the Company. The options will have a ten-year
term and will be subject to customary terms and conditions set forth in a stock
option agreement that we will provide to you. In addition to this initial grant,
the Board of Directors would also plan to make annual stock option grants and/or
other equity awards to you based on the performance of
Georges Gemayel
May 21, 2008
Page 2

the Company, commensurate with your position and consistent with market equity
grant practices and the then existing stock option plan approved by the
shareholders as administered in the discretion of the Compensation Committee.

In accordance with the terms of the enclosed Severance and Change in Control
Agreement, you will also be entitled to severance at a rate equal to your
then-current base salary (1) for twenty-four (24) months in the event Altus
terminates your employment without Cause or you resign for Good Reason within
one year following a Change in Control, or (2) for twelve (12) months in
the event Altus terminates your employment without Cause or you resign for Good
Reason in other circumstances. You will be entitled to two (2) times your
then-current target bonus in the event Altus terminates your employment without
Cause or you resign for Good Reason within one year following a Change in
Control. In the event Altus terminates your employment without Cause or you
resign for Good Reason in other circumstances, you will be eligible, in the
discretion of the Company, to receive a Separation Bonus not to exceed your
target annual bonus for the year of termination prorated for the portion of the
year you were employed. Your severance benefits will also include health
insurance benefit continuation as set forth in the agreement and a customary
non-competition covenant during the term of the applicable severance period.

As used in this letter, “Cause”, “Change in Control” and “Good Reason” have the
meanings set forth in our customary stock option agreement and your Severance
and Change in Control Agreement

Altus offers a competitive and comprehensive benefits program. All employees are
eligible to participate in the program, which includes group health, dental,
life, and long and short-term disability insurance. You are also eligible to
participate in our matching 401k plan, in accordance with plan guidelines. You
will also be entitled to four weeks of paid vacation annually, personal time and
observed as well as floating holidays.

Your offer of employment is subject to your agreement to Altus’ standard
employee non-disclosure and inventions agreement and the Severance and Change in
Control Agreement. We have included a copy of these agreements with this offer.
Please bring the signed copies with you on your first day.

Your offer of employment is also subject to completion of a customary background
check. Please sign and return the consent forms so that we can initiate the
process. In addition, the Federal government requires that we verify the
employment eligibility of all new employees. On your first day, please bring two
forms of identification (valid driver’s license, original birth certificate,
passport, original social security card, etc) demonstrating you are legally
eligible to work in the United States. This information is required to complete
the INS (Immigration and Naturalization Service) form I-9.

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Georges Gemayel
May 21, 2008
Page 3

We hope you find the Altus offer and position attractive and look forward to
your favorable response. Please return one copy of this letter indicating your
acceptance to me.

Sincerely,

/s/ David Pendergast
David Pendergast, Ph.D.
Executive Chairman

I accept the terms of employment offered in this letter.

Signature: /s/ Georges Gemayel

Date: May 21, 2008

Commencement Date: June 2, 2008

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