There are both DNA and RNA viruses.

G(uanine), C(ytosine) and U(racil) are RNA nucleotides, so it seemed more appropriate somehow...

I missed the fact that you were referring to "GCU". That was a little too subtle for my simple mind. :)

And retroviruses- an RNA virus that reverse transcribes itself into a DNA provirus: http://en.wikipedia.org/wiki/Reverse_transcribing_virus