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: Immunohistochemistry with monoclonal antibodies against neurofilament ( NF ) proteins of middle and high molecular weight class , NF - M and NF - H , was used to study axonal injury in the borderzone of focal lesions in rats .
### Output Text:
Immunohistochemistry with monoclonal antibodies against neurofilament ( NF ) proteins of middle and high molecular weight class , NF - M and NF - H , was used to study axonal injury in the borderzone of focal lesions in rats . 