The BRAF^V600Emutation is detectable in mature and immature hematopoietic cells from patients with histiocytosis. (A) Percentage of patients for whom BRAF^V600E mutation was detected in mature PB cells. (B) BRAF^V600E mutant allele frequency in fluorescence-activated cell sorting–purified cells from the BM or PB of 5 histiocytosis patients. ND indicates populations not studied. Patient numbering per supplemental Table 1. (C) Percentage of patients for whom the BRAF^V600E mutation was detected in CD34⁺ BM cells or in CD34-derived CFU-M, CFU-G, BFU-E, and plasmacytoid DCs (pDC) or mDCs. (D) BRAF^V600E, TET2^L1819X, and SRSF2^L95P status of 20 colonies derived from single CD34⁺ BM cells of a patient with BRAF^V600E-mutant MH (#P10). The results suggest that histiocytosis and AMML cells of this patient derived from a common TET2-mutated HSPC. In panels A and C, each denominator indicates the total number of patients for whom results were obtained.