If A Cell Is NOT Equal to a list of values OR Empty

Im trying to add a list section to my IF Statement that says if a cell isnt equal to AA, BB, CC or Empty then 'Do something'

Ive got a SQL Background so I tried the following below

ElseIf .Range("C1").Value IS NOT IN ("AA","BB","CC","") Then

...

But that gives me a syntax error. Ive not found anything online to help at the moment so I wondered if anyone had any tips on how you would write this line?..

Excellent, That worked a treat, thank you !!!!