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: BACKGROUND : A transient leukoencephalopathy mimicking cerebrovascular accident has been described as a complication of chemotherapy , most commonly in recipients of intrathecal methotrexate for childhood leukaemia .
### Output Text:
BACKGROUND : A transient leukoencephalopathy mimicking cerebrovascular accident has been described as a complication of chemotherapy , most commonly in recipients of intrathecal <span class="chemical">methotrexate</span> for childhood leukaemia . 