Summary of comparative genomics of ALL in identical twin pairs. The figure is based on analysis of 12 monozygotic twin pairs (the number of pairs with each founder lesion is noted in parentheses) with concordant ALL 29,33,37,40,48. The sharing of a patient- and clone-specific fusion gene that is not inherited in the twins indicates that in such cases of concordant ALL, the leukaemia must have been initiated in a single cell, in one twin of the pair in utero, the clonal progeny of that cell then disseminating to the co-twin, via intra-placental anastomoses. In further support of this notion, it was noted that concordance of ALL only occurred where the twins shared a single or monochorionic placenta, providing a route for cellular transmission 29. Figure adapted from reference 189. CNAs, copy number alterations. SNVs, single nucleotide variants.