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: Recently , many research groups have reported on the positive relationship between the genetic variations in the DRD2 gene and the therapeutic response in schizophrenia patients as a result of the role of variations in the receptor in modulating receptor expression .
### Output Text:
Recently , many research groups have reported on the positive relationship between the genetic variations in the DRD2 gene and the therapeutic response in schizophrenia patients as a result of the role of variations in the receptor in modulating receptor expression . 