Rare cancers: a sea of opportunity Niki Boyd , PhD Prof Janet E Dancey Prof C Blake Gilks , MD Prof David G Huntsman , MD Volume 17, No. 2 , e52–e61, February 2016 Summary Rare cancers, as a collective, account for around a quarter of all cancer diagnoses and deaths. Historically, they have been divided into two groups: cancers defined by their unusual histogenesis (cell of origin or differentiation state)—including chordomas or adult granulosa cell tumours—and histologically defined subtypes of common cancers. Most tumour types in the first group are still clinically and biologically relevant, and have been disproportionately important as sources of insight into cancer biology. By contrast, most of those in the second group have been shown to have neither defining molecular features nor clinical utility. Omics-based analyses have splintered common cancers into a myriad of molecularly, rather than histologically, defined subsets of common cancers, ...