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: All - trans retinoic acid ( ATRA ) , a component of standard therapy for acute promyelocytic leukemia ( APL ) , is associated with potentially serious but treatable adverse effects involving numerous organ systems , including rare skeletal muscle involvement .
### Output Text:
<span class="chemical">All - trans retinoic acid</span> ( <span class="chemical">ATRA</span> ) , a component of standard therapy for acute promyelocytic leukemia ( APL ) , is associated with potentially serious but treatable adverse effects involving numerous organ systems , including rare skeletal muscle involvement . 