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: Improvement and eventually full recovery only occurred after TAC was completely discontinued and successfully replaced by everolimus .
### Output Text:
Improvement and eventually full recovery only occurred after <span class="chemical">TAC</span> was completely discontinued and successfully replaced by <span class="chemical">everolimus</span> . 