[R] Question about range of letters

[Nia Gupta]

Hello, 

I have a column with a bunch of letters. I would like to keep some of these letters (A,C,D,L) and turn the rest into 'X'. 

I have tried using ifelse with '|' in between the argument but it didn't work nor did 4 separate ifelse statements. 

Example, I currently have:
Letters    A    B    C    D    E
I would like to have:
Letters    A    X    C    D    X
Thank you

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