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: This study investigates whether polymorphisms in the HOMER1 gene promoter region are associated with the occurrence of the chronic complications of levodopa therapy .
### Output Text:
This study investigates whether polymorphisms in the HOMER1 gene promoter region are associated with the occurrence of the chronic complications of <span class="chemical">levodopa</span> therapy . 