Guarnerio et al. show that oncogenic translocations also give rise to fusion-circular RNAs (f-circRNAs). The authors went on to demonstrate that f-circRNAs derived from the mixed lineage leukaemia (MLL)–AF9 (also known as MLLT3) fusion gene were able to induce transformation of immortalized cells in vitro and of haematopoietic stem cells expressing the MLL–AF9 fusion protein in vivo. Furthermore, they found that these f-circRNAs conferred resistance of MLL–AF9-expressing leukaemia cells to therapy, and knockdown of MLL–AF9-derived f-circRNAs increased apoptosis and the expression of p27 and p21 in THP1 cells (which have the MLL–AF9 translocation), indicating that f-circRNAs are also important for cell viability.