Papaya ringspot virus (PRSV), a member of the family Potyviridae, is a positive-sense single-stranded RNA plant virus with a monocistronic genome of c. 10 326 nt that is expressed via a large polyprotein subsequently cleaved into functional proteins. It causes severe damage on cucurbit crops such as squash and melons, and worldwide damage on papaya, a widely grown tropical fruit crop. Leaves of infected plants show mosaic symptoms and are often distorted and shoestring in shape, and infected plants are stunted. Fruit from infected plants can be malformed and show bumps, and infected papaya fruit often show ringspot symptoms, from which the name is derived. PRSV is grouped into type P or type W biotypes based on their host range. The P biotype infects cucurbits and papaya, while the W biotype infects only cucurbits. The virus is transmitted in a nonpersistent manner by a number of aphid species. Genetically engineered papaya expressing the coat protein gene of PRSV are resistant to the virus and are used commercially in Hawaii, one of only two cases for using commercial transgenic virus-resistant crops in the US.