

<!-- RR Donnelley Xcelerate Taxonomy Schema, based on XBRL 2.1  http://www.rrdonnelley.com/ -->
<!-- Version: 6.22.4 -->
<!-- Round: 6 -->
<!-- Creation date: 2015-05-12T03:38:41Z -->
<!-- Copyright (c) 2005-2013 R.R. Donnelley & Sons Company All Rights Reserved. -->
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        <link:definition>101 - Document - Document and Entity Information</link:definition>
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        <link:definition>108 - Disclosure - Summary of Significant Accounting Policies</link:definition>
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        <link:definition>109 - Disclosure - Fair Value Measurements</link:definition>
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        <link:definition>110 - Disclosure - Collaboration and Licensing Agreements</link:definition>
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        <link:definition>111 - Disclosure - Stock-Based Compensation</link:definition>
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        <link:definition>112 - Disclosure - Net Loss Per Common Share</link:definition>
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        <link:definition>113 - Disclosure - Shareholders Equity</link:definition>
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        <link:definition>114 - Disclosure - Subsequent Events</link:definition>
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        <link:definition>115 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
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        <link:definition>117 - Disclosure - Stock-Based Compensation (Tables)</link:definition>
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        <link:definition>119 - Disclosure - Fair Value Measurements - Additional Information (Detail)</link:definition>
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        <link:definition>120 - Disclosure - Fair Value Measurements - Summary of Fair Value Measurements of Company's Financial Assets (Detail)</link:definition>
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        <link:definition>121 - Disclosure - Collaboration and Licensing Agreements - Additional Information (Detail)</link:definition>
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        <link:definition>122 - Disclosure - Stock-Based Compensation - Summary of Stock-Based Compensation Expense Recognized for Stock Options and Employee Stock Purchase Plan (ESPP) (Detail)</link:definition>
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        <link:definition>123 - Disclosure - Stock-Based Compensation - Additional Information (Detail)</link:definition>
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        <link:definition>124 - Disclosure - Net Loss Per Common Share - Computation of Basic and Diluted Earnings Per Common Share (Detail)</link:definition>
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        <link:definition>125 - Disclosure - Net Loss Per Common Share - Additional Information (Detail)</link:definition>
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        <link:definition>126 - Disclosure - Stockholders Equity - Additional Information (Detail)</link:definition>
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        <link:definition>127 - Disclosure - Subsequent Events - Additional Information (Detail)</link:definition>
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