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: CONCLUSIONS : Delayed - onset heparin - induced thrombocytopenia is increasingly being recognized .
### Output Text:
CONCLUSIONS : Delayed - onset <span class="chemical">heparin</span> - induced thrombocytopenia is increasingly being recognized . 