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: MPA , BCC , DMCM , and STR showed no inhibition of 3H - TBOB ( t - butyl bicyclo - orthobenzoate ) binding at concentrations of 100 micron .
### Output Text:
<span class="chemical">MPA</span> , <span class="chemical">BCC</span> , <span class="chemical">DMCM</span> , and <span class="chemical">STR</span> showed no inhibition of <span class="chemical">3H - TBOB</span> ( <span class="chemical">t - butyl bicyclo - orthobenzoate</span> ) binding at concentrations of 100 micron . 