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: METHODS : This is a cross - sectional study involving 95 patients with PD on uninterrupted levodopa therapy for at least 6 months .
### Output Text:
METHODS : This is a cross - sectional study involving 95 patients with PD on uninterrupted <span class="chemical">levodopa</span> therapy for at least 6 months . 