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 appearance of MPO - ANCA in these cases was suspected of being related to PTU because the titres of MPO - ANCA decreased when PTU was stopped .
### Output Text:
The appearance of MPO - ANCA in these cases was suspected of being related to <span class="chemical">PTU</span> because the titres of MPO - ANCA decreased when <span class="chemical">PTU</span> was stopped . 