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: At the time of this writing , dose level 4 ( 260 , 270 , and 280 mg / m2 ) is being evaluated ; three patients from this level are evaluable .
### Output Text:
At the time of this writing , dose level 4 ( 260 , 270 , and 280 mg / m2 ) is being evaluated ; three patients from this level are evaluable . 