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: Northern blot analysis of hepatic RNA revealed a dose - dependent increase in the expression of a number of genes critical for fatty acid oxidation , lipoprotein assembly , and lipid transport .
### Output Text:
Northern blot analysis of hepatic RNA revealed a dose - dependent increase in the expression of a number of genes critical for <span class="chemical">fatty acid</span> oxidation , lipoprotein assembly , and lipid transport . 