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: Pregnenolone sulphate ( PREGS ) has generated interest as one of the most potent memory - enhancing neurosteroids to be examined in rodent learning studies , with particular importance in the ageing process .
### Output Text:
<span class="chemical">Pregnenolone sulphate</span> ( <span class="chemical">PREGS</span> ) has generated interest as one of the most potent memory - enhancing neurosteroids to be examined in rodent learning studies , with particular importance in the ageing process . 