Identification of FNDC3B as a new fusion partner of RARA in a variant APL. (A) Left, May-Grünwald-Giemsa staining showing 2 abnormal promyelocytes in the diagnostic BM aspirate. The abnormal cells in the BM were medium-sized and showed round, indented, or occasionally bilobed nuclei, dispersed chromatin, prominent nucleoli, and abundant heavily granulated cytoplasm. Auer rod was rarely found. Right, Sudan Black B (SBB) staining on BM aspirate showing 2 SBB-positive abnormal promyelocytes. Original magnification ×1000. (B) A karyotype performed on the diagnostic BM revealed 45,X,-Y,t(3;17)(q26;q21). The breakpoint regions of derivative chromosomes 3 and 17 are marked with arrows. (C) Left, Interphase FISH using the PML/RARA dual-color dual-fusion translocation probes revealed splitting of the RARA gene. Two diminished split RARA signals (green) are marked with arrows. Right, Metaphase FISH analysis confirmed 3′-RARA location at 3q26 (arrowhead). 5′-RARA remained at the long arm of derivative chromosome 17 (arrow). (D) Partial nucleotide sequences surrounding the junctions of the FNDC3B-RARA and RARA-FNDC3B fusions. RARA-FNDC3B (1) and RARA-FNDC3B (2) represent the major and minor reciprocal fusion transcripts, respectively. (E) Schematic diagram of FNDC3B, RARA, FNDC3B-RARA, and RARA-FNDC3B fusion proteins. The breakpoint is indicated by a red line. The blue area in FNDC3B is a putative transmembrane domain. The shaded area in RARA-FNDC3B (2) indicates novel sequences resulting from an out-of-frame fusion. The numbers indicate amino acid positions. (F) RT-PCR analysis of the full-length FNDC3B-RARA and RARA-FNDC3B fusion transcripts in BM collected at diagnosis (AD) and molecular remission (MR). Amplification of GAPDH served as an internal control. DBD, DNA-binding domain; LBD, ligand-binding domain; NTC, no template control.