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: BACKGROUND : Direct thrombin inhibitors ( DTIs ) provide an alternative method of anticoagulation for patients with a history of heparin - induced thrombocytopenia ( HIT ) or HIT with thrombosis ( HITT ) undergoing cardiopulmonary bypass ( CPB ) .
### Output Text:
BACKGROUND : Direct thrombin inhibitors ( DTIs ) provide an alternative method of anticoagulation for patients with a history of <span class="chemical">heparin</span> - induced thrombocytopenia ( HIT ) or HIT with thrombosis ( HITT ) undergoing cardiopulmonary bypass ( CPB ) . 