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: Myocardial accumulation of [ 125I ] MIBG 4 hr after intravenous injection did not differ between the controls and the groups treated 3 wk or less .
### Output Text:
Myocardial accumulation of [ 125I ] <span class="chemical">MIBG</span> 4 hr after intravenous injection did not differ between the controls and the groups treated 3 wk or less . 