Definition: In this task, you're given a paragraph from the research paper and your task is to generate a suitable title for the research paper based on the given paper. Under 100 words is a good title length.
Input: Retroviruses are classified as exogenous and endogenous retroviruses according to the mode of transmission. Endogenous retroviruses (ERVs) are retroviruses which have been integrated into germ-line cells and inherited from parents to offspring. Most ERVs are inactivated by deletions and mutations; however, certain ERVs maintain their infectivity and infect the same host and new hosts as exogenous retroviruses. All domestic cats have infectious ERVs, termed RD-114 virus. Several canine and feline attenuated vaccines are manufactured using RD-114 virus-producing cell lines such as Crandell-Rees feline kidney cells; therefore, it is possible that infectious RD-114 virus contaminates live attenuated vaccines. Recently, Japanese and UK research groups found that several feline and canine vaccines were indeed contaminated with infectious RD-114 virus. This was the first incidence of contamination of 'infectious' ERVs in live attenuated vaccines. RD-114 virus replicates efficiently in canine cell lines and primary cells. Therefore, it is possible that RD-114 virus infects dogs following inoculation with contaminated vaccines and induces proliferative diseases and immune suppression, if it adapts to grow efficiently in dogs. In this review, we summarize the incidence of contamination of RD-114 virus in live attenuated vaccines and potential risks of infection with RD-114 virus in dogs. Retroviruses enter host cells via binding of the envelope proteins with the host receptor(s). After entering the cells, viral RNA is reverse-transcribed into DNA and then the DNA is integrated into the host genome to be a provirus. Viral RNA is transcribed from the provirus and the structural and enzymatic proteins of the virus are synthesized from the transcribed viral RNA. Retroviruses are classified as exogenous and endogenous retroviruses according to the mode of transmission [14] . Usually, exogenous retroviruses infect somatic cells but not germ-line cells, and they are transmitted horizontally by infection via viral particles. On the other hand, ERVs are retroviruses which have been integrated into germ-line cells. ERVs behave like normal genes and are inherited from parents to offspring as Mendelian's law. ERVs occupy about 10 % of mammalian genomes and are mostly inactivated by deletions and mutations [14, 15, 17, 27] . However, a limited number of ERVs maintain their infectivity and infect new hosts as exogenous retroviruses [6] . Exogenous retroviruses are classified into seven genera, i.e., alpharetrovirus, betaretrovirus, gammaretrovirus, deltaretrovirus, epsilonretrovirus, spumaretrovirus and Electronic supplementary material The online version of this article (

Contamination of live attenuated vaccines with an infectious feline endogenous retrovirus (RD-114 virus)