The knowledge of virus reproduction is necessary to design new safe drugs for inhibition of infections. Ultra-violet irradiation of virus proteins with labeled virus genome fragments permits to identify specific nucleic acid binding proteins. Affinity modification of enzymes with nucleotide derivatives could help to determine NTP-binding proteins and those involved in viral genome replication. Photoreactive analogues of nucleic acids are among the tools used to detect elongation subunits of replicative complexes. Affinity modification approach has already resulted in successful treatment of virus diseases.