In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Challenge tests with AX were performed in 23 subjects ( 43 % ) to establish the diagnosis of immediate allergic reaction to AX , and in 15 cases ( 28 % ) both skin test and RAST for AX were negative .
### Output Text:
Challenge tests with <span class="chemical">AX</span> were performed in 23 subjects ( 43 % ) to establish the diagnosis of immediate allergic reaction to <span class="chemical">AX</span> , and in 15 cases ( 28 % ) both skin test and RAST for <span class="chemical">AX</span> were negative . 