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: DNA from peripheral nuclear blood cells was genotyped for 5 - HTTLPR using standard polymerase chain reaction methods . A significant group effect was observed only on memory function tasks ( p = 0 . 04 ) but not on reaction times ( p = 0 . 61 ) or attention / executive functioning ( p = 0 . 59 ) .
### Output Text:
DNA from peripheral nuclear blood cells was genotyped for 5 - HTTLPR using standard polymerase chain reaction methods . A significant group effect was observed only on memory function tasks ( p = 0 . 04 ) but not on reaction times ( p = 0 . 61 ) or attention / executive functioning ( p = 0 . 59 ) . 