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: Malondialdehyde level in BCNU - exposed group was higher than those in control groups and melatonin decreased malondialdehyde levels in BCNU group ( P < 0 . 01 ) , while there were no significant differences in the superoxide dismutase levels between these groups .
### Output Text:
<span class="chemical">Malondialdehyde</span> level in <span class="chemical">BCNU</span> - exposed group was higher than those in control groups and <span class="chemical">melatonin</span> decreased <span class="chemical">malondialdehyde</span> levels in <span class="chemical">BCNU</span> group ( P < 0 . 01 ) , while there were no significant differences in the <span class="chemical">superoxide</span> dismutase levels between these groups . 