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: Urine NAG activity is thus a measure of altered function in the renal tubules and not simply an indicator of damage .
### Output Text:
Urine NAG activity is thus a measure of altered function in the renal tubules and not simply an indicator of damage . 