In 1970, Hoggan reported the detection of latent infections of AAV in human and monkey kidney cells. He and his coworkers screened cell lines intended for vaccine production and found that approximately 1% of human embryonic kidney cells and 20% of African green monkey kidney cells produced AAV when infected with helper adenovirus. These results suggested that AAV stayed in an integrated form in the absence of helper virus and that this inapparent infection was a rather frequent natural occurrence. Hoggan et al. (1972) could prove by using Detroit 6 cells that a deliberate infection with AAV in the absence of helper virus led to a latent infection. A superinfection with helper adenovirus led to the induction of AAV replication.