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: Earlier studies revealed the major role of Extracellular signal - regulated kinase ( ERK ) pathways in low - density lipoprotein receptor ( LDLr ) expression .
### Output Text:
Earlier studies revealed the major role of Extracellular signal - regulated kinase ( ERK ) pathways in low - density lipoprotein receptor ( LDLr ) expression . 