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: " Real - world " data on the efficacy and safety of lenalidomide and dexamethasone in patients with relapsed / refractory multiple myeloma who were treated according to the standard clinical practice : a study of the Greek Myeloma Study Group .
### Output Text:
" Real - world " data on the efficacy and safety of <span class="chemical">lenalidomide</span> and <span class="chemical">dexamethasone</span> in patients with relapsed / refractory multiple myeloma who were treated according to the standard clinical practice : a study of the Greek Myeloma Study Group . 