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: Agranulocytosis occurred 3 - 20 weeks after initiation of ticlopidine , so frequent examination of white cell count during treatment is recommended .
### Output Text:
Agranulocytosis occurred 3 - 20 weeks after initiation of <span class="chemical">ticlopidine</span> , so frequent examination of white cell count during treatment is recommended . 