Understanding the genetic origins of the patient's underlying oncologic diagnoses by comparing somatic mutations from respective tumor samples. To understand the genetic origins of this patient’s LCH, HLH, and AML diagnoses, we compared somatic mutations in respective tumor samples using a skin biopsy sample collected at the time of AML diagnosis as a normal control (A). A formalin-fixed, paraffin-embedded sample of spleen was used, representative of the LCH/HLH lesion. The bone marrow and skin biopsy samples were collected at the time of AML diagnosis. The skin biopsy was used as a matched germline control. Whole-exome sequencing was performed for all these samples with a mean read depth of ×200 to ×300. Illumina paired-end reads were preprocessed and mapped to the human reference genome (hg38). We used an ensemble approach to call somatic mutations (SNV/indels) with 5 published tools. Consensus by at least 2 callers were considered confident mutations and were further manually reviewed for the read depth, mapping quality, and strand bias to remove additional artifacts. The AML sample was also independently analyzed using the St. Jude clinical genomics platform.¹⁵  Both LCH and AML shared the BRAF:p.V600E somatic mutation that was previously reported by the histopathology analysis, with an elevated tumor variant allele frequency in the AML sample (B-C). Furthermore, the AML sample harbored additional somatic mutations characteristic of AML and absent in the LCH sample, including monosomy 7, KRAS:p.G60D, NRAS:p.T58delinsILDT, EZH2:p.E740fs, IKZF1:c.197-5T>C, and RUNX1::POU2F2 fusion. Additional somatic mutations of unclear significance were identified in both LCH and AML samples (B-C). These somatic mutations likely arose after the initial LCH diagnosis and during AML development. This analysis indicated that the residual cells after initial therapy containing BRAF mutations expanded to LCH/HLH relapse and again into AML while acquiring additional AML driver mutations in the process (D). AraC, cytarabine; 2CdA,– cladribine; 6MP, mercaptopurine; MTX, methotrexate; VAF, variant allele frequency; VP16, etoposide. Figure created using BioRender.com.