Interphase FISH to confirm CNAs identified by SNP array in twin set 5. Panels A and B show leukemic blasts from twins 5a and 5b, respectively, hybridized with the ETV6-RUNX1 ES (Vysis extra signal) probe and a fosmid probe (W12-1173N4) for the TBL1XR1 gene. The TBL1XR1 probe is labeled with biotin-16-dUTP and detected with streptavidin Cy5 (here pseudocolored pink). In panel A, the signal pattern of leukemic blasts in twin 5a is 1 yellow (corresponding to the ETV6-RUNX1 gene fusion), 2 red (1 corresponding to the normal RUNX1 gene and 1 to the residual RUNX1 gene disrupted by the translocation), and 2 pink (TBL1XR1 gene) signals. Note that the second green signal is not present, indicating loss of the normal (unrearranged) ETV6 gene. This pattern was observed in 21 of 31 cells scored. The other 10 of 31 cells had a normal pattern. In panel B, the signal pattern of leukemic blasts in twin 5b is 1 yellow (ETV6-RUNX1 gene fusion), 1 green (unrearranged ETV6), 3 red (corresponding to an additional copy of the RUNX1 gene), and 1 pink (corresponding to 1 copy of the TBL1XR1 gene). This accords with the SNP data indicating 3 discordant “driver” CNAs in this twin pair involving the ETV6, RUNX1, and TBL1XR1 genes. This pattern was observed in 31 of 32 cells scored.