Chain letters (3)
I believe this clue is a double definition.
'chain' is the first definition.
(I've seen this before)
'letters' is the second definition.
(I've seen this in another clue)
(Other definitions for mail that I've seen before include "Letters - armour" , "Eg, letters" , "Armour; posted items" , "Paper" , "Form of armour; post" .)
(Another definition for rna that I've seen is " Messenger".)