GMODP frequency in blood reports response to therapy in 1 case study. (A) The frequencies (%) of (G)MODP cells among total live cells in PBMC samples of 23 LCH patients (supplemental Table 1) collected at first disease onset or relapse (prior to (n = 20) or after the start of systemic therapy (n = 3)) and age-matched HDs (n = 36), as determined by flow cytometry. Error bars indicate median and interquartile range. Indicated P value was determined by Mann-Whitney U test (**P < .01). The gray area indicates the calculated progenitor frequency range based on HD data. (B) Frequencies (%) of (G)MODP cells among live cells in the blood of MS-RO⁺ patient LCH_002 at diagnosis (before therapy [pre Tx]), 1 month after starting chemotherapy (on Tx), and 3 years after finishing salvage chemotherapy (post Tx) and an age-matched HD. (C-H) (G)MODP cells sorted from blood samples of LCH_002 taken before, during, or after therapy and an age-matched HD were seeded at 1000 cells per well and cultured under LC-differentiation conditions. (C) Light microscopic appearance of the cultures. (D) Absolute number of live cells per well at the end of the culture period. (E) Flow cytometry plots depicting the gating strategy for sorting of (G)MODP offspring cells for BRAF mutation analysis. (F) Flow cytometry plots depicting CD207⁺CD1a⁺ LC-like cells within HLA-DR⁺CD11c⁺CD14⁺ offspring cells. (G-H) DNA was extracted from the G₁, G₂, and G₃ fractions of the cultured cells, sorted as depicted in panel E and analyzed for the presence of the BRAF mutation by ddPCR. (G) Representative 2-dimensional ddPCR plot depicting BRAF^V600E mutation in (G)MODP-derived HLA-DR⁺CD14^-CD11c⁺ offspring cells from LCH_002. (H) BRAF^V600E mutation status as detected by ddPCR of (G)MODP offspring cells of the indicated phenotypes (sorted as indicated in panel E). (I) Flow cytometry plots depicting the gating strategy for sorting of DCs and Mo’s from blood samples of LCH_002. (J) Corresponding BRAF^V600E mutation status of DCs and Mo’s identified by the listed markers.