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: These data also suggest that the measurement of tissue GSSG may be a more sensitive indicator of oxidant stress than measurement of products of lipid peroxidation .
### Output Text:
These data also suggest that the measurement of tissue <span class="chemical">GSSG</span> may be a more sensitive indicator of oxidant stress than measurement of products of lipid peroxidation . 