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: Grade 3 - 4 neutropenia and thrombocytopenia were documented in four and three patients ( 17 and 13 % ) , respectively , but drug dose reduction due to cytopenia was not required in any case .
### Output Text:
Grade 3 - 4 neutropenia and thrombocytopenia were documented in four and three patients ( 17 and 13 % ) , respectively , but drug dose reduction due to cytopenia was not required in any case . 