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: Use of cidofovir in 1 child was associated with progressive renal failure and neutropenia .
### Output Text:
Use of <span class="chemical">cidofovir</span> in 1 child was associated with progressive renal failure and neutropenia . 