BM samples before (A and B) and during (C and D) As₂O₃ treatment were collected for analyzing the origin of differentiated myeloid cells. Morphological examination (A and C) showed promyelocytes (A) and myelocyte-like cells (C), in which one red, one green, and one yellow fusion signal could be observed in the same cells (B and D). Arrows pointed yellow signals that represented PML- RAR fusion gene.