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: Similarly , the total protein abundance of the Na - K - 2Cl cotransporter ( NKCC2 ) in the medulla but not in the cortex of the + / + mice was significantly reduced by lithium treatment .
### Output Text:
Similarly , the total protein abundance of the <span class="chemical">Na</span> - <span class="chemical">K</span> - <span class="chemical">2Cl</span> cotransporter ( NKCC2 ) in the medulla but not in the cortex of the + / + mice was significantly reduced by <span class="chemical">lithium</span> treatment . 