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: Sequential biological assays of IL - 1 , IL - 2 , TNF and PAF were performed during Ara - C infusion to ten patients , including the four who developed the syndrome .
### Output Text:
Sequential biological assays of IL - 1 , IL - 2 , TNF and PAF were performed during <span class="chemical">Ara - C</span> infusion to ten patients , including the four who developed the syndrome . 