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: The findings of this study suggest that lesions of the unilateral STN and GPi are equally effective treatment for patients with advanced PD refractory to medical treatment .
### Output Text:
The findings of this study suggest that lesions of the unilateral STN and GPi are equally effective treatment for patients with advanced PD refractory to medical treatment . 