The direction which RNA polymerase moves during transcription (5' to 3') and ribosomes moves during translation. By convention, the +1 position of a gene is the first transcribed nucleotide, nucleotides downstream from the +1 position are designated +2, +3, etc. Drift evolution: A high rate of immunologically significant mutations in certain viruses. This results in drifting away from recognition by the immune system by antigenic change. Influenza virus, HIV and HCV constantly change their antigenic structure through drift evolution.