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: Using in situ hybridization probes for the alpha3 and alpha5 subunits , we showed that alpha5 mRNA levels are unchanged , whereas alpha3 mRNA levels are selectively decreased in the mitral cell layer of the olfactory bulb , and the inferior and the superior colliculus of beta4 - / - brains .
### Output Text:
Using in situ hybridization probes for the alpha3 and alpha5 subunits , we showed that alpha5 mRNA levels are unchanged , whereas alpha3 mRNA levels are selectively decreased in the mitral cell layer of the olfactory bulb , and the inferior and the superior colliculus of beta4 - / - brains . 