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: A gene cluster comprises the alpha3 , alpha5 and beta4 subunits , which coassemble to form functional receptors .
### Output Text:
A gene cluster comprises the alpha3 , alpha5 and beta4 subunits , which coassemble to form functional receptors . 