As an alternative to sequencing, Proudnikov et al. [12] applied a hybridization-based technique to the detection of genetic variants of poliovirus within a virus population or among viral strains. Oligonucleotide probes are synthesized and then immobilized on a solid surface. A target consisting of amplified viral complementary DNA (cDNA) then labeled and hybridized to the immobilized probes and the hybridization to the individual probes detected [12] . The presence of a change in the nucleotide sequence in the target is detected by the absence or the reduction of hybridization to the wild type probes around the change, or by the ratio of the signals generated by a mutant against a reference strain. Modifications of the above technique including the use of amplified viral complementary RNA (cRNA) were used to identify genetic variations arising during cultivation of a vaccine strain of poliovirus and the emergence of vaccine derived poliovirus in immunized patients showing signs of vaccine associated paralytic poliomyelitis [13, 14] . Application of this procedure was restricted, however, to identifying known mutations in specific virus strains.