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 / AIMS : Experimental studies have suggested that apoptosis via the Fas / Fas Ligand signaling system may play an important role in the development of acute liver failure .
### Output Text:
BACKGROUND / AIMS : Experimental studies have suggested that apoptosis via the Fas / Fas Ligand signaling system may play an important role in the development of acute liver failure . 