Flow cytometric and FISH evaluation of mast cells. (A) Detection of phenotypically abnormal mast cells (dim CD2⁺/CD25⁺) by difference from normal flow cytometry (ΔN). Gridlines are set based on autofluorescence of mast cells. (B) Swimmer plot showing duration of the fusion-positive mast cells for all patients (y-axis) in the cohort by number of days since the patient’s first known negative MRD specimen by ΔN flow cytometry (x-axis). All patients were MRD-negative by ΔN flow cytometry for the duration of testing. The red dots indicate time points with positive CBF fusion in the mast cells alone and the green dots indicate specimens that had no detectable CBF fusions in any cell population by FISH. PerCP, peridinin-chlorophyll-protein; PE, phycoerythrin; APC, allophycocyanin; FITC, fluorescein isothiocyanate; mono, monocytes; prog, progenitors.