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: Nevertheless , there have been no studies on the temporal relationship between the appearance of MPO - ANCA and vasculitis during PTU therapy , or on the incidence of MPO - ANCA in untreated Graves ' disease patients .
### Output Text:
Nevertheless , there have been no studies on the temporal relationship between the appearance of MPO - ANCA and vasculitis during <span class="chemical">PTU</span> therapy , or on the incidence of MPO - ANCA in untreated Graves ' disease patients . 