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: The treatment of HIT mandates an immediate cessation of all heparin exposure and the institution of an antithrombotic therapy , most commonly using a direct thrombin inhibitor .
### Output Text:
The treatment of HIT mandates an immediate cessation of all <span class="chemical">heparin</span> exposure and the institution of an antithrombotic therapy , most commonly using a <span class="chemical">direct thrombin inhibitor</span> . 