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: Radiolabeled metaiodobenzylguanidine ( MIBG ) , an analog of norepinephrine ( NE ) , serves as an index of adrenergic neuron integrity and function .
### Output Text:
<span class="chemical">Radiolabeled metaiodobenzylguanidine</span> ( <span class="chemical">MIBG</span> ) , an analog of <span class="chemical">norepinephrine</span> ( <span class="chemical">NE</span> ) , serves as an index of adrenergic neuron integrity and function . 