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: High - dose methotrexate ( HD - MTX ) is an important treatment for Burkitt lymphoma , but can cause hepatic and renal toxicity when its clearance is delayed .
### Output Text:
High - dose <span class="chemical">methotrexate</span> ( HD - <span class="chemical">MTX</span> ) is an important treatment for Burkitt lymphoma , but can cause hepatic and renal toxicity when its clearance is delayed . 