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: Skin tests with AX were positive in 34 ( 63 % ) patients .
### Output Text:
Skin tests with <span class="chemical">AX</span> were positive in 34 ( 63 % ) patients . 