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: The clinical and laboratory data of 27 consecutive children who underwent LT from January 2007 to December 2010 in our center were analyzed retrospectively .
### Output Text:
The clinical and laboratory data of 27 consecutive children who underwent LT from January 2007 to December 2010 in our center were analyzed retrospectively . 