**EXHIBIT 15.1**

 

August 1, 2006

 

The Board of Directors and Stockholders of RigelPharmaceuticals, Inc.

 

We are aware ofthe incorporation by reference in the following Registration Statements:

 
| (1)    |   | Form S-3 No. 333-129650, of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                           |
|--------|--------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| (2)    |   | Form S-3 No. 333-119785, of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                           |
| (3)    |   | Form S-3 No. 333-112746, of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                           |
| (4)    |   | Form S-3 No. 333-111777, of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                           |
| (5)    |   | Form S-3 No. 333-106942, of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                           |
| (6)    |   | Form S-3 No. 333-105431, of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                           |
| (7)    |   | Form S-3 No. 333-87276 of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                             |
| (8)    |   | Form S-3 No. 333-74906 of Rigel  Pharmaceuticals, Inc.                                                                                                                                                                                                                                                                                                             |
| (9)    |   | Form S-8 No. 333-125895, pertaining to the  2000 Equity Incentive Plan, 2000 Employee Stock Purchase Plan and 2000  Non-Employee Directors’ Stock Option Plan of Rigel Pharmaceuticals, Inc.                                                                                                                                                                  |
| (10)   |   | Form S-8 No. 333-111782, pertaining to the  2000 Equity Incentive Plan of Rigel Pharmaceuticals, Inc.                                                                                                                                                                                                                                                              |
| (11)   |   | Form S-8 No. 333-107062, pertaining to the  2000 Employee Stock Purchase Plan of Rigel Pharmaceuticals, Inc.                                                                                                                                                                                                                                                       |
| (12)   |   | Form S-8 No. 333-106532, pertaining to the  2000 Equity Incentive Plan, 2000 Employee Stock Purchase Plan and 2000  Non-Employee Directors’ Stock Option Plan of Rigel Pharmaceuticals, Inc.                                                                                                                                                                  |
| (13)   |   | Form S-8 No. 333-72492, pertaining to the  2001 Non-Officer Equity Incentive Plan of Rigel Pharmaceuticals, Inc.                                                                                                                                                                                                                                                   |
| (14)   |   | Form S-8 No. 333-51184 pertaining to the  2000 Equity Incentive Plan, 2000 Employee Stock Purchase Plan and 2000 Non-Employee  Directors’ Stock Option Plan of Rigel Pharmaceuticals, Inc.                                                                                                                                                                    |
|   |   |                                                                                                                                                                                                                                                                                                                                                                              |
|   |   | in each case related to the sale of shares of common  stock, and in the related prospectuses, as applicable, contained in such  Registration Statements of our report dated April 24, 2006, relating to the  unaudited condensed interim financial statements of Rigel  Pharmaceuticals, Inc. that are included in its Form 10-Q for the  quarter ended March 31, 2006. |
|   |   |                                                                                                                                                                                                                                                                                                                                                                              |
|   |   | Pursuant to Rule 436(c) of the Securities  Act of 1933, our report is not a part of the registration statements prepared  or certified by accountants within the meaning of section 7 or 11 of the  Securities Act of 1933, as amended.                                                                                                                            |

 
|                  | /s/ Ernst & Young LLP |   |
|-----------------------|---------------------------|--------|
|                  |                      |
| Palo Alto, California |                      |
| August 1, 2006        |                      |

 

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