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 variety of factors , such as vasoactive properties of tacrolimus , proliferation of Demodex due to local immunosuppression , and the occlusive properties of the ointment , may be involved in the observed phenomena .
### Output Text:
A variety of factors , such as vasoactive properties of <span class="chemical">tacrolimus</span> , proliferation of Demodex due to local immunosuppression , and the occlusive properties of the ointment , may be involved in the observed phenomena . 