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 frequencies of HIT after liver transplantation and platelet factor 4 / heparin - reactive antibody ( HIT antibody ) positivity in liver transplantation patients , however , are unknown .
### Output Text:
The frequencies of HIT after liver transplantation and platelet factor 4 / <span class="chemical">heparin</span> - reactive antibody ( HIT antibody ) positivity in liver transplantation patients , however , are unknown . 