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: The mechanism of this myopathy is uncertain but may involve the induction by statins of an endoplasmic reticulum stress response with associated up - regulation of MHC - I expression and antigen presentation by muscle fibres .
### Output Text:
The mechanism of this myopathy is uncertain but may involve the induction by <span class="chemical">statins</span> of an endoplasmic reticulum stress response with associated up - regulation of MHC - I expression and antigen presentation by muscle fibres . 