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 : The increased concentration of sFas in serum of patients with acute liver failure may reflect activation of Fas - mediated apoptosis in the liver and this together with increased tumor necrosis factor - alpha may be an important factor in liver cell loss .
### Output Text:
CONCLUSIONS : The increased concentration of sFas in serum of patients with acute liver failure may reflect activation of Fas - mediated apoptosis in the liver and this together with increased tumor necrosis factor - alpha may be an important factor in liver cell loss . 