In vivo maturation of t(11;17) FISH-positive leukemia cells. Twelve days after initiation of ATRA and G-CSF treatment, FISH-positive cells in bone marrow and blood predominantly showed segmented nuclei (as visualized by DAPI staining) indicative of granulocytic differentiation of the leukemic cells. To establish the morphology of the FISH-positive cells, slides were stained with May-Grünwald-Giemsa (A). The same fields were photographed after hybridization of the slides with labeled FISH probes (B) to obtain dual morphological and FISH staining. The t(11;17) translocation is indicated by the colocalization of red and green signals.