How to negate inside a case

I'm a bit new in plsql, so could you tell me it exists a way to negate a case? I mean, this way:

 select CASE DENOM_CURRENCY_CODE
            WHEN != 'MXN' THEN 'convert to USD'
            ELSE TO_CHAR(QUANTITY) --keep the quantity
        END 

I received this messaage "ORA-00936: missing expression" I´ve tried too with "<>" and "not". Thanks for reading