2006 Employee Stock Purchase Plan

Exhibit 10.24


ACHILLION PHARMACEUTICALS, INC.

2006 EMPLOYEE STOCK PURCHASE PLAN

The purpose of this Plan is to provide eligible employees of Achillion Pharmaceuticals, Inc. (the " Company" ) and certain of its subsidiaries with opportunities to purchase shares of the Company' s common stock, $.001 par value (the " Common Stock" ). 2,000,000 shares of Common Stock in the aggregate have been approved for this purpose. This Plan is intended to qualify as an " employee stock purchase plan" as defined in Section 423 of the Internal Revenue Code of 1986, as amended (the " Code" ), and the regulations promulgated thereunder, and shall be interpreted consistent therewith.


1. Administration . The Plan will be administered by the Company' s Board of Directors (the " Board" ) or by a Committee appointed by the Board (the " Committee" ). The Board or the Committee has authority to make rules and regulations for the administration of the Plan and its interpretation and decisions with regard thereto shall be final and conclusive.


2. Eligibility . All employees of the Company, including Directors who are employees, and all employees of any subsidiary of the Company (as defined in Section 424(f) of the Code) designated by the Board or the Committee from time to time (a " Designated Subsidiary" ), are eligible to participate in any one or more of the offerings of Options (as defined in Section 9) to purchase Common Stock under the Plan provided that:

(a) they are customarily employed by the Company or a Designated Subsidiary for more than 20 hours a week and for more than five months in a calendar year; and

(b) they have been employed by the Company or a Designated Subsidiary for at least six months prior to enrolling in the Plan; and


(c) they are employees of the Company or a Designated Subsidiary on the first day of the applicable Plan Period (as defined below).


No employee may be granted an option hereunder if such employee, immediately after the option is granted, owns 5% or more of the total combined voting power or value of the stock of the Company or any subsidiary. For purposes of the preceding sentence, the attribution rules of Section 424(d) of the Code shall apply in determining the stock ownership of an employee, and all stock which the employee has a contractual right to purchase shall be treated as stock owned by the employee.


3. Offerings . The Company will make one or more offerings (" Offerings" ) to employees to purchase stock under this Plan. Offerings will begin each September 1 and March 1, or the first business day thereafter (the " Offering Commencement Dates" ). Each Offering Commencement Date will begin a six-month period (a " Plan Period" ) during which payroll deductions will be made and held for the purchase of Common Stock at the end of the Plan Period. The Board or the Committee may, at its discretion, choose a different Plan Period of twelve (12) months or less for subsequent Offerings. Notwithstanding anything to the contrary, the first Plan Period shall begin on the later of (i) the first date that the Common Stock is


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publicly traded following the Company' s initial public offering (the " IPO Date" ) or (ii) September 1, 2006, and shall end on February 28, 2007.


4. Participation . An employee eligible on the Offering Commencement Date of any Offering may participate in such Offering by completing and forwarding a payroll deduction authorization form to the employee' s appropriate payroll office at least five business days prior to the applicable Offering Commencement Date. The form will authorize a regular payroll deduction from the Compensation received by the employee during the Plan Period. Unless an employee files a new form or withdraws from the Plan, his deductions and purchases will continue at the same rate for future Offerings under the Plan as long as the Plan remains in effect. The term " Compensation" means the amount of money reportable on the employee' s Federal Income Tax Withholding Statement, excluding overtime, shift premium, incentive or bonus awards, allowances and reimbursements for expenses such as relocation allowances for travel expenses, income or gains associated with the grant or vesting of restricted stock, income or gains on the exercise of Company stock options or stock appreciation rights, and similar items, whether or not shown on the employee' s Federal Income Tax Withholding Statement, but including, in the case of salespersons, sales commissions to the extent determined by the Board or the Committee.


5. Deductions . The Company will maintain payroll deduction accounts for all participating employees. With respect to any Offering made under this Plan, an employee may authorize a payroll deduction in any dollar amount up to a maximum of 15% of the Compensation he or she receives during the Plan Period or such shorter period during which deductions from payroll are made. The Board or the Committee may, at its discretion, designate a lower maximum contribution rate. Payroll deductions may be at the rate of 2%, 4%, 6%, 8% or 10% of Compensation with any change in compensation during the Plan Period to result in an automatic corresponding change in the dollar amount withheld. The minimum payroll deduction is such percentage of compensation as may be established from time to time by the Board or the Committee.

6. Deduction Changes . An employee may decrease or discontinue his payroll deduction once during any Plan Period, by filing a new payroll deduction authorization form. However, an employee may not increase his payroll deduction during a Plan Period. If an employee elects to discontinue his payroll deductions during a Plan Period, but does not elect to withdraw his funds pursuant to Section 8 hereof, funds deducted prior to his election to discontinue will be applied to the purchase of Common Stock on the Exercise Date (as defined below).


7. Interest . Interest will not be paid on any employee accounts, except to the extent that the Board or the Committee, in its sole discretion, elects to credit employee accounts with interest at such per annum rate as it may from time to time determine.

8. Withdrawal of Funds . An employee may at any time prior to the close of business on the last business day in a Plan Period and for any reason permanently draw out the balance accumulated in the employee' s account and thereby withdraw from participation in an Offering. Partial withdrawals are not permitted. The employee may not begin participation again during


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the remainder of the Plan Period. The employee may participate in any subsequent Offering in accordance with terms and conditions established by the Board or the Committee.


9. Purchase of Shares .

(a) Number of Shares . On the Offering Commencement Date of each Plan Period, the Company will grant to each eligible employee who is then a participant in the Plan an option (an " Option" ) to purchase on the last business day of such Plan Period (the " Exercise Date" ) at the applicable purchase ...