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: Cessation of TDF was associated with complete restoration of kidney function in up half of the patients in this report .
### Output Text:
Cessation of <span class="chemical">TDF</span> was associated with complete restoration of kidney function in up half of the patients in this report . 