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: In a placebo - controlled , single - blinded , crossover study , we assessed the effect of " real " repetitive transcranial magnetic stimulation ( rTMS ) versus " sham " rTMS ( placebo ) on peak dose dyskinesias in patients with Parkinson ' s disease ( PD ) .
### Output Text:
In a placebo - controlled , single - blinded , crossover study , we assessed the effect of " real " repetitive transcranial magnetic stimulation ( rTMS ) versus " sham " rTMS ( placebo ) on peak dose dyskinesias in patients with Parkinson ' s disease ( PD ) . 