All letters, most of them in the high range of the alphabet
[hint]8 is the only hint I can think of :p[/hint]
Spoiler
After trying regular ciphers like Atbash, ROT, Vigenere and noticing that the letters where mostly in the upper range of the alphabet I converted them to numbers (a=0) 22 13 13 3 11 23 12 21 15 21 11 11 23 21 24 11 21 23 3 12 22 11 21 12 13 23 12 3 21 22 24 12 22 21 22 21
If you add up all the digits you get a sum of 128. This divided by 8 gives 16 which makes it a good candidate for binary run length encoding. Knowing also that numbers in 8 bit binary start with 00 and the fact that the first number is a 2 makes it even more obvious.
