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: Three ( 4 . 1 % ) of the 73 patients were positive for MPO - ANCA at 13 , 16 and 17 months , respectively , after the start of PTU therapy .
### Output Text:
Three ( 4 . 1 % ) of the 73 patients were positive for MPO - ANCA at 13 , 16 and 17 months , respectively , after the start of <span class="chemical">PTU</span> therapy . 