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                                                                  EXHIBIT 10.5.1

                           ALTUS PHARMACEUTICALS INC.

                        INCENTIVE STOCK OPTION AGREEMENT

      AGREEMENT made as of the <<AGREEMENT_DATE>>, between Altus Pharmaceuticals
Inc. (the "Company"), a DelawarE corporation having a principal place of
business in Cambridge, Massachusetts, and <<FIRST>> <<LAST>>, an employee oF the
Company (the "Employee").

      WHEREAS, the Company desires to grant to the Employee an Option to
purchase shares of its common stock, $.01 par value per share (the "Shares"),
under and for the purposes set forth in the Company's Amended and Restated 2002
Employee, Director and Consultant Stock Plan (the "Plan");

      WHEREAS, the Company and the Employee understand and agree that any terms
used and not defined herein have the same meanings as in the Plan; and

      WHEREAS, the Company and the Employee each intend that the Option granted
herein qualify as an ISO.

      NOW, THEREFORE, in consideration of the mutual covenants hereinafter set
forth and for other good and valuable consideration, the parties hereto agree as
follows:

      1. GRANT OF OPTION.

      The Company hereby grants to the Employee the right and option to purchase
all or any part of an aggregate of <<OPTIONS>> Shares, on the terms and
conditions and subject to all the limitations set forth herein, under United
States securities and tax laws, and in the Plan, which is incorporated herein by
reference. The Employee acknowledges receipt of a copy of the Plan.

      2. PURCHASE PRICE.

      The purchase price of the Shares covered by the Option shall be
$<<PRICE>> per Share, subject to adjustment, as provided in the Plan, in the
event of a stock split, reverse stock split or other events affecting the
holders of Shares after the date hereof (the "Purchase Price"). Payment shall be
made in accordance with Paragraph 9 of the Plan.

      3. EXERCISABILITY OF OPTION.

      Subject to the terms and conditions set forth in this Agreement and the
Plan, the Option granted hereby shall become exercisable as follows:

On <<VESTING_ANNIVERSARY_DATE>> and every three months   One-sixteenth of the
   Shares thereafter until the fourth anniversary of
    <<VESTING_DATE>>

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      The foregoing rights are cumulative and are subject to the other terms and
conditions of this Agreement and the Plan.

      Notwithstanding the above, in the event within one year after the date of
a Change in Control (as defined in Paragraph 24B of the Plan), (i) the Employee
is terminated as an employee by the Company or an Affiliate for any reason other
than Cause (as defined in the Plan), or (ii) the Employee as a condition to his
or her remaining an employee of the Company or an Affiliate, is required to
relocate at least 50 miles from his or her current location of employment, or
(iii) there occurs a material adverse change in the Employee's duties, authority
or responsibilities which causes the Employee's position with the Company to
become of significantly less responsibility or authority than the Employee's
position was immediately prior to the Change in Control, or (iv) there occurs a
reduction in the Employee's base salary from the Employee's base salary received
immediately prior to the Change in Control, this Option will be immediately
exercisable as of the date of the Employee's last day of employment, unless this
Option has otherwise expired or been terminated pursuant to its terms or the
terms of the Plan.

      4. TERM OF OPTION.

      The Option shall terminate ten years from the date of this Agreement or,
if the Employee owns as of the date hereof more than 10% of the total combined
voting power of all classes of capital stock of the Company or an Affiliate,
five years from the date of this Agreement, but shall be subject to earlier
termination as provided herein or in the Plan.

      If the Employee ceases to be an employee of the Company or of an Affiliate
(for any reason other than the death or Disability of the Employee or
termination of the Employee's employment for "cause" (as defined in the Plan),
the Option may be exercised, if it has not previously terminated, within three
months after the date the Employee ceases to be an employee of the Company or an
Affiliate, or within the originally prescribed term of the Option, whichever is
earlier, but may not be exercised thereafter. In such event, the Option shall be
exercisable only to the extent that the Option has become exercisable and is in
effect at the date of such cessation of employment.

      Notwithstanding the foregoing, in the event of the Employee's Disability
or death within three months after the termination of employment, the Employee
or the Employee's Survivors may exercise the Option within one year after the
date of the Employee's termination of employment, but in no event after the date
of expiration of the term of the Option.

      In the event the Employee's employment is terminated by the Employee's
employer for "cause" (as defined in the Plan), the Employee's right to exercise
any unexercised portion of this Option shall cease immediately as of the time
the Employee is notified his or her employment is terminated for "cause," and
this Option shall thereupon terminate. Notwithstanding anything herein to the
contrary, if subsequent to the Employee's termination as an employee, but prior
to the exercise of the Option, the Board of Directors of the Company determines
that, either prior or subsequent to the Employee's termination, the Employee
engaged in conduct which would constitute "cause," then the Employee shall
immediately cease to have any right to exercise the Option and this Option shall
thereupon terminate.

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      In the event of the Disability of the Employee, as determined in
accordance with the Plan, the Option shall be exercisable within one year after
the Employee's termination of employment or, if earlier, within the term
originally prescribed by the Option. In such event, the Option shall be
exercisable:

      (a)   to the extent that the Option has become exercisable but has not
            been exercised as of the date of Disability; and

      (b)   in the event rights to exercise the Option accrue periodically, to
            the extent of a pro rata portion through the date of Disability of
            any additional vesting rights that would have accrued on the next
            vesting date had the Employee not become Disabled. The proration
            shall be based upon the number of days accrued in the current
            vesting period prior to the date of Disability.

      In the event of the death of the Employee while an employee of the Company
or of an Affiliate, the Option shall be exercisable by the Employee's Survivors
within one year after the date of death of the Employee or, if earlier, within
the originally prescribed term of the Option. In such event, the Option shall be
exercisable:

      (x)   to the extent that the Option has become exercisable but has not
            been exercised as of the date of death; and

      (y)   in the event rights to exercise the Option accrue periodically, to
            the extent of a pro rata portion through the date of death of any
            additional vesting rights that would have accrued on the next
            vesting date had the Employee not died. The proration shall be based
            upon the number of days accrued in the current vesting period prior
            to the Employee's date of death.

      5. METHOD OF EXERCISING OPTION.

      Subject to the terms and conditions of this Agreement, the Option may be
exercised by written notice to the Company or its designee, in substantially the
form of Exhibit A attached hereto. Such notice shall state the number of Shares
with respect to which the Option is being exercised and shall be signed by the
person exercising the Option. Payment of the purchase price for such Shares
shall be made in accordance with Paragraph 9 of the Plan. The Company shall
deliver such Shares as soon as practicable after the notice shall be received,
provided, however, that the Company may delay issuance of such Shares until
completion of any action or obtaining of any consent, which the Company deems
necessary under any applicable law (including, without limitation, state
securities or "blue sky" laws). The Shares as to which the Option shall have
been so exercised shall be registered in the Company's share register in the
name of the person so exercising the Option (or, if the Option shall be
exercised by the Employee and if the Employee shall so request in the notice
exercising the Option, shall be registered in the name of the Employee and
another person jointly, with right of survivorship) and shall be delivered as
provided above to or upon the written order of the person exercising the Option.
In the event the Option shall be exercised, pursuant to Section 4 hereof, by any
person other than the Employee, such notice shall be accompanied by appropriate
proof of the right of

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such person to exercise the Option. All Shares that shall be purchased upon the
exercise of the Option as provided herein shall be fully paid and nonassessable.

      6. PARTIAL EXERCISE.

      Exercise of this Option to the extent above stated may be made in part at
any time and from time to time within the above limits, except that no
fractional share shall be issued pursuant to this Option.

      7. NON-ASSIGNABILITY.

      The Option shall not be transferable by the Employee otherwise than by
will or by the laws of descent and distribution. The Option shall be
exercisable, during the Employee's lifetime, only by the Employee (or, in the
event of legal incapacity or incompetency, by the Employee's guardian or
representative) and shall not be assigned, pledged or hypothecated in any way
(whether by operation of law or otherwise) and shall not be subject to
execution, attachment or similar process. Any attempted transfer, assignment,
pledge, hypothecation or other disposition of the Option or of any rights
granted hereunder contrary to the provisions of this Section 7, or the levy of
any attachment or similar process upon the Option shall be null and void.

      8. NO RIGHTS AS STOCKHOLDER UNTIL EXERCISE.

      The Employee shall have no rights as a stockholder with respect to Shares
subject to this Agreement until registration of the Shares in the name of the
Employee. Except as is expressly provided in the Plan with respect to certain
changes in the capitalization of the Company, no adjustment shall be made for
dividends or similar rights for which the record date is prior to the date of
such registration.

      9. ADJUSTMENTS.

      The Plan contains provisions covering the treatment of Options in a number
of contingencies such as stock splits and mergers. Provisions in the Plan for
adjustment with respect to stock subject to Options and the related provisions
with respect to successors to the business of the Company are hereby made
applicable hereunder and are incorporated herein by reference.

      10. TAXES.

      The Employee acknowledges that any income or other taxes due from him or
her with respect to this Option or the Shares issuable pursuant to this Option
shall be the Employee's responsibility.

      In the event of a Disqualifying Disposition (as defined in Section 15
below) or if the Option is converted into a Non-Qualified Option and such
Non-Qualified Option is exercised, the Company may withhold from the Employee's
remuneration, if any, the minimum statutory

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amount of federal, state and local withholding taxes attributable to such amount
that is considered compensation includable in such person's gross income. At the
Company's discretion, the amount required to be withheld may be withheld in cash
from such remuneration, or in kind from the Shares otherwise deliverable to the
Employee on exercise of the Option. The Employee further agrees that, if the
Company does not withhold an amount from the Employee's remuneration sufficient
to satisfy the Company's income tax withholding obligation, the Employee will
reimburse the Company on demand, in cash, for the amount under-withheld.

      11. PURCHASE FOR INVESTMENT.

      Unless the offering and sale of the Shares to be issued upon the
particular exercise of the Option shall have been effectively registered under
the Securities Act of 1933, as now in force or hereafter amended (the "1933
Act"), the Company shall be under no obligation to issue the Shares covered by
such exercise unless and until the following conditions have been fulfilled:

      (a)   The person(s) who exercise the Option shall warrant to the Company,
            at the time of such exercise, that such person(s) are acquiring such
            Shares for their own respective accounts, for investment, and not
            with a view to, or for sale in connection with, the distribution of
            any such Shares, in which event the person(s) acquiring such Shares
            shall be bound by the provisions of the following legend which shall
            be endorsed upon the certificate(s) evidencing the Shares issued
            pursuant to such exercise:

                  "The shares represented by this certificate have been taken
                  for investment and they may not be sold or otherwise
                  transferred by any person, including a pledgee, unless (1)
                  either (a) a Registration Statement with respect to such
                  shares shall be effective under the Securities Act of 1933, as
                  amended, or (b) the Company shall have received an opinion of
                  counsel satisfactory to it that an exemption from registration
                  under such Act is then available, and (2) there shall have
                  been compliance with all applicable state securities laws;"
                  and

      (b)   If the Company so requires, the Company shall have received an
            opinion of its counsel that the Shares may be issued upon such
            particular exercise in compliance with the 1933 Act without
            registration thereunder. Without limiting the generality of the
            foregoing, the Company may delay issuance of the Shares until
            completion of any action or obtaining of any consent, which the
            Company deems necessary under any applicable law (including without
            limitation state securities or "blue sky" laws).

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      12. RESTRICTIONS ON TRANSFER OF SHARES.

      12.1 The Employee agrees that in the event the Company proposes to offer
for sale to the public any of its equity securities and such Employee is
requested by the Company and any underwriter engaged by the Company in
connection with such offering to sign an agreement restricting the sale or other
transfer of Shares, then it will promptly sign such agreement and will not
transfer, whether in privately negotiated transactions or to the public in open
market transactions or otherwise, any Shares or other securities of the Company
held by him or her during such period as is determined by the Company and the
underwriters, not to exceed 90 days following the closing of the offering, plus
such additional period of time as may be required to comply with Marketplace
Rule 2711 of the National Association of Securities Dealers, Inc. or similar
rules thereto (such period, the "Lock-Up Period"). Such agreement shall be in
writing and in form and substance reasonably satisfactory to the Company and
such underwriter and pursuant to customary and prevailing terms and conditions.
Notwithstanding whether the Employee has signed such an agreement, the Company
may impose stop-transfer instructions with respect to the Shares or other
securities of the Company subject to the foregoing restrictions until the end of
the Lock-Up Period.

      12.2 The Employee acknowledges and agrees that neither the Company, its
shareholders nor its directors and officers, has any duty or obligation to
disclose to the Employee any material information regarding the business of the
Company or affecting the value of the Shares before, at the time of, or
following a termination of the employment of the Employee by the Company,
including, without limitation, any information concerning plans for the Company
to make a public offering of its securities or to be acquired by or merged with
or into another firm or entity.

      13. NO OBLIGATION TO EMPLOY.

      The Company is not by the Plan or this Option obligated to continue the
Employee as an employee of the Company or an Affiliate. The Employee
acknowledges: (i) that the Plan is discretionary in nature and may be suspended
or terminated by the Company at any time; (ii) that the grant of the Option is a
one-time benefit which does not create any contractual or other right to receive
future grants of options, or benefits in lieu of options; (iii) that all
determinations with respect to any such future grants, including, but not
limited to, the times when options shall be granted, the number of shares
subject to each option, the option price, and the time or times when each option
shall be exercisable, will be at the sole discretion of the Company; (iv) that
the Employee's participation in the Plan is voluntary; (v) that the value of the
Option is an extraordinary item of compensation which is outside the scope of
the Employee's employment contract, if any; and (vi) that the Option is not part
of normal or expected compensation for purposes of calculating any severance,
resignation, redundancy, end of service payments, bonuses, long-service awards,
pension or retirement benefits or similar payments.

      14. OPTION IS INTENDED TO BE AN ISO.

      The parties each intend that the Option be an ISO so that the Employee (or
the Employee's Survivors) may qualify for the favorable tax treatment provided
to holders of

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Options that meet the standards of Section 422 of the Code. Any provision of
this Agreement or the Plan which conflicts with the Code so that this Option
would not be deemed an ISO is null and void and any ambiguities shall be
resolved so that the Option qualifies as an ISO. Nonetheless, if the Option is
determined not to be an ISO, the Employee understands that neither the Company
nor any Affiliate is responsible to compensate him or her or otherwise make up
for the treatment of the Option as a Non-qualified Option and not as an ISO. The
Employee should consult with the Employee's own tax advisors regarding the tax
effects of the Option and the requirements necessary to obtain favorable tax
treatment under Section 422 of the Code, including, but not limited to, holding
period requirements.

      15. NOTICE TO COMPANY OF DISQUALIFYING DISPOSITION.

      The Employee agrees to notify the Company in writing immediately after the
Employee makes a Disqualifying Disposition of any of the Shares acquired
pursuant to the exercise of the Option. A Disqualifying Disposition is defined
in Section 424(c) of the Code and includes any disposition (including any sale)
of such Shares before the later of (a) two years after the date the Employee was
granted the Option or (b) one year after the date the Employee acquired Shares
by exercising the Option, except as otherwise provided in Section 424(c) of the
Code. If the Employee has died before the Shares are sold, these holding period
requirements do not apply and no Disqualifying Disposition can occur thereafter.

      16. NOTICES.

      Any notices required or permitted by the terms of this Agreement or the
Plan shall be given by recognized courier service, facsimile, registered or
certified mail, return receipt requested, addressed as follows:

If to the Company:

                           Altus Pharmaceuticals Inc.
                           125 Sidney Street
                           Cambridge, MA 02139
                           Attn:  Legal Department

If to the Employee:

                           <<First>> <<Last>>
                           <<Address>>
                           <<City>>, <<State>> <<Zip>>

or to such other address or addresses of which notice in the same manner has
previously been given. Any such notice shall be deemed to have been given upon
the earlier of receipt, one business day following delivery to a recognized
courier service or three business days following mailing by registered or
certified mail.

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      17. GOVERNING LAW.

      This Agreement shall be construed and enforced in accordance with the law
of the State of Delaware, without giving effect to the conflict of law
principles thereof. For the purpose of litigating any dispute that arises under
this Agreement, the parties hereby consent to exclusive jurisdiction in
Massachusetts and agree that such litigation shall be conducted in the courts of
Middlesex County, Massachusetts or the federal courts of the United States for
the District of Massachusetts.

      18. BENEFIT OF AGREEMENT.

      Subject to the provisions of the Plan and the other provisions hereof,
this Agreement shall be for the benefit of and shall be binding upon the heirs,
executors, administrators, successors and assigns of the parties hereto.

      19. ENTIRE AGREEMENT.

      This Agreement, together with the Plan, embodies the entire agreement and
understanding between the parties hereto with respect to the subject matter
hereof and supersedes all prior oral or written agreements and understandings
relating to the subject matter hereof. No statement, representation, warranty,
covenant or agreement not expressly set forth in this Agreement shall affect or
be used to interpret, change or restrict, the express terms and provisions of
this Agreement, provided, however, in any event, this Agreement shall be subject
to and governed by the Plan.

      20. MODIFICATIONS AND AMENDMENTS.

      The terms and provisions of this Agreement may be modified or amended as
provided in the Plan.

      21. WAIVERS AND CONSENTS.

      Except as provided in the Plan, the terms and provisions of this Agreement
may be waived, or consent for the departure therefrom granted, only by written
document executed by the party entitled to the benefits of such terms or
provisions. No such waiver or consent shall be deemed to be or shall constitute
a waiver or consent with respect to any other terms or provisions of this
Agreement, whether or not similar. Each such waiver or consent shall be
effective only in the specific instance and for the purpose for which it was
given, and shall not constitute a continuing waiver or consent.

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      22. DATA PRIVACY.

      By entering into this Agreement, the Employee: (i) authorizes the Company
and each Affiliate, and any agent of the Company or any Affiliate administering
the Plan or providing Plan recordkeeping services, to disclose to the Company or
any of its Affiliates such information and data as the Company or any such
Affiliate shall request in order to facilitate the grant of options and the
administration of the Plan; (ii) waives any data privacy rights he or she may
have with respect to such information; and (iii) authorizes the Company and each
Affiliate to store and transmit such information in electronic form.

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      IN WITNESS WHEREOF, the Company has caused this Agreement to be executed
by its duly authorized officer, and the Employee has hereunto set his or her
hand, all as of the day and year first above written.

                                    ALTUS PHARMACEUTICALS INC.

                                    By:_________________________________________
                                       Name: <<Signer>>
                                       Title: <<Title>>

                                    ____________________________________________
                                    <<First>> <<Last>>

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                                                                       Exhibit A

                  NOTICE OF EXERCISE OF INCENTIVE STOCK OPTION

IMPORTANT NOTICE: This form of Notice of Exercise may only be used at such time
as the Company has filed a Registration Statement with the Securities and
Exchange Commission under which the issuance of the Shares for which this
exercise is being made is registered and such Registration Statement remains
effective.

TO: ALTUS PHARMACEUTICALS INC.

Ladies and Gentlemen:

      I hereby exercise my Incentive Stock Option to purchase _________ shares
(the "Shares") of the common stock, $.01 par value, of Altus Pharmaceuticals
Inc. (the "Company"), at the exercise price of $________ per share, pursuant to
and subject to the terms of that certain Incentive Stock Option Agreement
between the undersigned and the Company dated _______________, 200_.

      I understand the nature of the investment I am making and the financial
risks thereof. I am aware that it is my responsibility to have consulted with
competent tax and legal advisors about the relevant national, state and local
income tax and securities laws affecting the exercise of the Option and the
purchase and subsequent sale of the Shares.

      I am paying the option exercise price for the Shares as follows:

                        ________________________________

      Please issue the Shares (check one):

      [ ] to me; or

      [ ] to me and ____________________________, as joint tenants with right of
                    survivorship,
      at the following address:

      _________________________________________

      _________________________________________

      _________________________________________

                                      A-1
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      My mailing address for shareholder communications, if different from the
address listed above, is:
      _________________________________________

      _________________________________________

      _________________________________________

                                      Very truly yours,

                                      __________________________________________
                                      Employee (signature)

                                      __________________________________________
                                      Print Name

                                      __________________________________________
                                      Date

                                      __________________________________________
                                      Social Security Number

                                      A-2