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: Analysis of 4HNE protein adducts by immunogold electron microscopy showed appearance of 4HNE protein adducts in mitochondria as early as 3 hours , with a peak at 6 hours and subsequent decline at 24 hours .
### Output Text:
Analysis of <span class="chemical">4HNE</span> protein adducts by immunogold electron microscopy showed appearance of <span class="chemical">4HNE</span> protein adducts in mitochondria as early as 3 hours , with a peak at 6 hours and subsequent decline at 24 hours . 