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: This supports ongoing concerns about intensive MTX exposure as a major contributor to CNS late effects .
### Output Text:
This supports ongoing concerns about intensive <span class="chemical">MTX</span> exposure as a major contributor to CNS late effects . 