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: [ Nat . Neurosci . 9 ( 2006 ) , 917 ] recently identified the microglial - specific fractalkine receptor ( CX3CR1 ) as an important mediator of MPTP - induced neurodegeneration of DA neurons .
### Output Text:
[ Nat . Neurosci . 9 ( 2006 ) , 917 ] recently identified the microglial - specific fractalkine receptor ( CX3CR1 ) as an important mediator of <span class="chemical">MPTP</span> - induced neurodegeneration of <span class="chemical">DA</span> neurons . 