These include ARID1B, ARID2, BCL11B, and SMARCA2 mutations in post-transplant lymphoproliferative disorders [189, 216], ARID1A mutations in primary histiocytic sarcoma [217], and ARID1A mutations in aggressive Langerhans cell histiocytosis [218]. The gene discussed is ARID1A; the disease is Langerhans cell histiocytosis.