Detection of PIK3CA^E542K mutation. (A) Sequenom mass-spectrometric–based genotyping assays. Mutants are detectable with the appearance of a new peak (gray arrows) with an allele frequency ∼25% in a lesion with 50% tumor infiltration. (B) Sanger sequencing. (C) Anti–phospho-AKT staining in the PIK3CA^E542K mutated LCH. Histiocytes from an LCH lesion of the patient with PIK3CA^542K mutation show positive cytoplasmic staining in comparison with histiocytes from 2 LCH lesions from patients without the PIK3CA^542K mutation (1 patient with the BRAF^V600E mutation and 1 patient without a known mutation).