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: A significant reduction in urinary guanosine 3 ' , 5 ' - cyclic monophosphate ( cGMP ) excretion and a significant increase in renal cortical renin and endothelin - 1 contents were also observed in GM - mediated nephropathy .
### Output Text:
A significant reduction in urinary <span class="chemical">guanosine 3 ' , 5 ' - cyclic monophosphate</span> ( <span class="chemical">cGMP</span> ) excretion and a significant increase in renal cortical renin and endothelin - 1 contents were also observed in <span class="chemical">GM</span> - mediated nephropathy . 