We describe a nucleic acid comprising the sequence RNNWCAAA, in which R is independently G or A, N is independently T, C, G or A and W is independently T or A, suitable for the treatment or prevention of hepatitis B or cancer. N at position 3 may be C, A or T, preferably A or T, more preferably T N at position 2 may be C W at position 4 may be T and R at position 1 may be A. The nucleic acid may have the sequence ACATCAAA or ACTTCAAA.
