FISH analysis with a probe set for MBCR andABL, in blasts from a patient with recently treated acute lymphoblastic leukemia shown to be Philadelphia-chromosome–negative and BCR/ABL-negative. The digoxigenin-labeled probe forBCR is detected with anti-dig rhodamine (red), and the biotinylated probe to ABL is detected with fluoreceinated avidin (green). The 3 illustrations represent cells in late S- (A and B) and G₂M-phase (C) of the cell cycle. In (A), there are 4 green and 3 red signals with close proximity of BCR/ABL(inset). In (B), there are 4 green and 3 red signals, including a doubling BCR (red) signal which is juxtaposed to ABL(inset). The cell in (C) is at G₂M and shows 4 green and 4 red signals with no juxtaposition of BCR and ABL.