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: Kinetic analysis showed that the p . Val204Asp / p . Asp70Gly - p . Ala539Thr BChE displays a pure Michaelian behavior with BTC as the substrate .
### Output Text:
Kinetic analysis showed that the p . Val204Asp / p . Asp70Gly - p . Ala539Thr BChE displays a pure Michaelian behavior with <span class="chemical">BTC</span> as the substrate . 