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: During the course of nephrotic syndrome , serum urea concentrations increased significantly faster in sgk1 ( - / - ) mice than in sgk1 ( + / + ) mice leading to uremia and a reduced median survival in sgk1 ( - / - ) mice ( 29 vs . 40 days in sgk1 ( + / + ) mice ) .
### Output Text:
During the course of nephrotic syndrome , serum <span class="chemical">urea</span> concentrations increased significantly faster in sgk1 ( - / - ) mice than in sgk1 ( + / + ) mice leading to uremia and a reduced median survival in sgk1 ( - / - ) mice ( 29 vs . 40 days in sgk1 ( + / + ) mice ) . 