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: Response and euthymia were defined , a priori , as at least a 50 % improvement from baseline to end point and as a score of no less than 12 at end point in the Y - MRS total score , respectively .
### Output Text:
Response and euthymia were defined , a priori , as at least a 50 % improvement from baseline to end point and as a score of no less than 12 at end point in the Y - MRS total score , respectively . 