add substitution for special character '$'

I came across another character with special meaning for screen:

``xy <- fdata450k$TargetID[!(fdata450k$CHR %in% c("X","Y"))]``
was sent to R like
``xy <- fdata450k[!(fdata450k %in% c("X","Y"))]``

I push the substitution while I try to find out if there are more special characters interpreted by screen (I couldn't find anything else in the documentation).

Thanks!
Sergi