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: Six patients ( 12 % ) had World Health Organization Grade 3 - 4 neutropenia , 2 patients ( 4 % ) had Grade 3 - 4 thrombocytopenia , and 2 patients ( 4 % ) had Grade 3 neurotoxicity .
### Output Text:
Six patients ( 12 % ) had World Health Organization Grade 3 - 4 neutropenia , 2 patients ( 4 % ) had Grade 3 - 4 thrombocytopenia , and 2 patients ( 4 % ) had Grade 3 neurotoxicity . 