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: Four patients developed transient microscopic hematuria at 400 , 450 , and 500 mg / m2 / d .
### Output Text:
Four patients developed transient microscopic hematuria at 400 , 450 , and 500 mg / m2 / d . 