Genome replication Many viruses have evolved their own specific enzymatic mechanisms to preferentially replicate virus nucleic acids at the expense of cellular molecules There is often sufficient specificity in virus polymerases to provide a target for a specific antiviral agent, The majority of these antiviral drugs function as polymerase substrate(i.e nucleoside/nucleotide) analogues The toxicity of these drugs varies considerably from some which are well tolerated(e.g. acyclovir) to others which are highly toxic(e.g. AZT(azidothymidine)