In this instance, properties of the virus other than its ability to replicate and cause cell destruction could be used to devise a detection system; for example, such cells as skin fibroblasts from human cases of Fanconi's anemia (Todaro et al., 1966) , Down's syndrome (Todaro and Martin, 1967) or Bloom's syndrome (German et al., 1965) may be sensitive indicators for transforming viruses.