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: Because proteinuria is a major predictive factor of poor transplantation outcome , many studies focused on this adverse event during the past years .
### Output Text:
Because proteinuria is a major predictive factor of poor transplantation outcome , many studies focused on this adverse event during the past years . 