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: Twenty - five patients with rheumatoid arthritis ( RA ) who developed toxicity while taking remission inducing drugs and 30 without toxicity were studied for possible associations with class I and II HLA antigens .
### Output Text:
Twenty - five patients with rheumatoid arthritis ( RA ) who developed toxicity while taking remission inducing drugs and 30 without toxicity were studied for possible associations with class I and II HLA antigens . 