**EXHIBIT 15.1**

 

May 1,2006

 

The Boardof Directors and Stockholders of Rigel Pharmaceuticals, Inc.

 

We are aware of theincorporation 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 |
| May 1, 2006           |

 

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