Retroperitoneal leiomyoma is a rare type of benign smooth muscle tumor almost exclusively found in women and with histopathological features similar to uterine leiomyomas. The pathogenesis of retroperitoneal leiomyoma is unclear and next to nothing is known about the cytogenetics and molecular genetics of the tumor. Here we present the first cytogenetically analyzed retroperitoneal leiomyoma. It had a t(10;17)(q22;q21) as the sole chromosomal abnormality. Using RNA-Sequencing and the 'grep' command to search the fastq files of the sequence data we found that the translocation resulted in fusion of the genes KAT6B (10q22) with KANSL1 (17q21). RT-PCR together with direct (Sanger) sequencing verified the presence of a KAT6B-KANSL1 fusion transcript. No reciprocal KANSL1-KAT6B transcript was amplified suggesting that it was either absent or unexpressed. The KAT6B-KANSL1 fusion transcript consists of exons 1 to 3 of KAT6B and exons 11 to 15 of KANSL1, is 3667 bp long, has a 1398 bp long open reading frame, and codes for a 466 amino acid residue protein. The corresponding KAT6B-KANSL1 protein contains the NEMM domain (including the linker histone H1/H5, domain H15) of KAT6B and the PEHE domain of KANSL1. The function of the fusion protein might be regulation of transcription with an affinity for chromatin (linker histone H1/H5) and interaction with the HAT domain of KAT8 (PEHE domain). The tumor expressed HMGA2 and HMGA1 even though 12q14-15 and 6p looked normal by G-banding analysis. The tumor also expressed MED12 in the absence of exon 2 mutations. Overall, the data show that the examined retroperitoneal leiomyoma resembles a subset of uterine leiomyomas in terms of histology and genetics.