Morphological and genetic investigation of the index RARA-aAPL case. (A) Wright staining of BM smear. (B) Myeloperoxidase chemical staining of BM smear. (C) Karyotype 46,XX,inv(17)(p12q21) of BM sample. (D-E) Schematic diagram of the STAT3 exon 21::RARA exon 3 (D) and RARA exon 9@c.1225::LTR40a (E) splices, reported by Arriba version 2.0.1 software with NGS WTS data. (F-G) Electrophoresis result of the RT-PCR product (F), a diagram indicating primer locations (G, upper), and the Sanger sequencing results (G, lower) of the STAT3::RARA::LTR40a tripartite fusion transcript. The bottom peaks in forward Sanger sequencing are due to the minor STAT3 exon 20::RARA exon 3 splice. (H-I) STAT3 intron 21::RARA intron 2 (H) and RARA exon 9::LTR40a (I) splices at the genomic level, as revealed by NGS WGS visualized using the IGV version 2.3.92. (J) Abundant STAT3::RARA::LTR40a tandem genomic splicing fragments captured through optical genome mapping analysis. The human GRCh37 annotation reference was used to annotate gene coordinates on chromosome 17.