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: Leukopenia was dose limiting : as many as 62 . 5 % of patients experienced grade 4 leukopenia .
### Output Text:
Leukopenia was dose limiting : as many as 62 . 5 % of patients experienced grade 4 leukopenia . 