Ruth Sager pioneered two fields of genetics, challenging orthodoxy and in so doing opening up new fields for investigation. Beginning in the 1950s, at a time in which geneticists generally maintained that all genes were located in the chromosome of the cell nucleus, Ruth Sager showed that chloroplasts possessed their own genome, and worked out the standard methods for studying chloroplast genes, and their interactions with nuclear genes. Then in the 1970s Sager turned her skills to studying cancer genetics; when attention was focused on the recent discovery of cancer‐causing ‘oncogenes’, Sager took a wholly different approach: she hypothesised the existence of tumour‐suppressor genes, provided evidence for their existence, and demonstrated that such genes were inactivated in tumours.