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 current work indicates the ability of PG - 9 to induce beneficial effects on cognitive processes and stimulate activity of NGF synthesis in astroglial cells .
### Output Text:
The current work indicates the ability of <span class="chemical">PG - 9</span> to induce beneficial effects on cognitive processes and stimulate activity of NGF synthesis in astroglial cells . 