Antiviral RNA silencing and the resistance gene-conferred defense response are major antiviral immune systems in plants. Several of the components involved have been genetically or biochemically identified in Arabidopsis thaliana. One powerful tool to dissect antiviral immune systems involves a reverse genetic approach that analyzes Arabidopsis mutant lines with impaired antiviral defense responses. In particular, to better understand the signaling networks involved in the resistance gene-conferred antiviral response in host plants, establishment of mutant lines carrying the homozygous mutant allele and antiviral resistance gene is required. The information on well-characterized defense-related signaling mutant alleles and the PCR-based genotyping method provided in this chapter allows the efficient selection of Arabidopsis mutant lines that can be used to study antiviral resistance signaling networks and resistance mechanisms.