In blastic plasmacytoid dendritic cell neoplasm (BPDCN), mutations (mostly loss-of-function) and deletions in ARID1A seem to be recurrent, although cohort sizes have been low (8 altered patients, combined N = 70) [111, 112, 173–176]. The gene discussed is ARID1A; the disease is CD4+/CD56+ hematodermic neoplasm.