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: Observation by electronic microscope revealed structural damage of podocytes ; the reduced expression level of podocyte marker genes , nephrin and podocin , was also detected by q - PCR .
### Output Text:
Observation by electronic microscope revealed structural damage of podocytes ; the reduced expression level of podocyte marker genes , nephrin and podocin , was also detected by q - PCR . 