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: Chromosome substitution strains ( CSS ) , in which a single chromosome from one inbred strain ( donor ) has been transferred onto a second strain ( host ) by repeated backcrossing , may be used to identify quantitative trait loci ( QTLs ) that contribute to seizure susceptibility .
### Output Text:
Chromosome substitution strains ( CSS ) , in which a single chromosome from one inbred strain ( donor ) has been transferred onto a second strain ( host ) by repeated backcrossing , may be used to identify quantitative trait loci ( QTLs ) that contribute to seizure susceptibility . 