What should some in place of the question mark (?) to complete the following letter cluster series ?

RD, JF, DJ,ZP, ?

XX

Explanation:

RD, JF, DJ,ZP, ?

The logic followed here is :

R → (-8) → J → (-6) → D → (-4) → Z → (-2) → X

D → (+2) → F → (+4) → J → (+6) → P → (+8) → X 

So, the letter-cluster that replaces the (?) to complete the series is "XX"