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: CONCLUSIONS : PTU therapy may be related to the appearance of MPO - ANCA , but MPO - ANCA does not appear to be closely related to vasculitis .
### Output Text:
CONCLUSIONS : <span class="chemical">PTU</span> therapy may be related to the appearance of MPO - ANCA , but MPO - ANCA does not appear to be closely related to vasculitis . 