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: We conclude that ATRA is a valid therapeutic choice for patients with APL , although the procoagulant tendency is not completely corrected .
### Output Text:
We conclude that <span class="chemical">ATRA</span> is a valid therapeutic choice for patients with APL , although the procoagulant tendency is not completely corrected . 