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: To this day , transcriptome analysis has been performed in a rat model of LID [ Neurobiol . Dis . , 17 ( 2004 ) , 219 ] but information regarding the proteome is still lacking .
### Output Text:
To this day , transcriptome analysis has been performed in a rat model of LID [ Neurobiol . Dis . , 17 ( 2004 ) , 219 ] but information regarding the proteome is still lacking . 