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: The use of cidofovir in severe adenovirus infection has been limited by adverse effects , the most significant of which is nephrotoxicity .
### Output Text:
The use of <span class="chemical">cidofovir</span> in severe adenovirus infection has been limited by adverse effects , the most significant of which is nephrotoxicity . 