Clinical and pathological manifestations of neutrophilic dermatoses. (A) Photograph of the erythematous, tender nodule on the outer right thigh of patient 2, arising 6 weeks after initiation of the FLT3 inhibitor quizartinib. (B) Fragment size analysis for FLT3 ITD and NPM1 insertion mutations. A representative analysis for FLT3 ITD and NPM1 insertion mutations using a PCR fragment size assay targeting FLT3 exon 15 and NPM1 exon 12 in genomic DNA extracted from the skin biopsy sample from patient 2. Arrows indicate the respective insertion mutations found in the patient but not in the normal control. (C) Histology of skin biopsy from patient 3. Clusters of neutrophils are present within subcutaneous fat, admixed with histiocytes. No blasts are identified (original magnification 1000×, images taken on Olympus BX41 microscope). (D) EGFR (7p) and MET (7q) FISH. Copy number analysis by FISH was performed on a skin biopsy formalin-fixed paraffin-embedded tissue section from patient 3, using probes specific for EGFR (green), which corresponds to chromosome 7p, and MET (red), which corresponds to chromosome 7q. The arrows indicate nuclei showing a single copy loss of MET or 7q, identified within the subcutaneous neutrophilic infiltrates.