MEF2D-rearranged ALL is characterized by older age at diagnosis (median, 14 years), mature B-cell leukemia morphology (large, densely basophilic, and heavily vacuolated leukemic blasts), a unique immunophenotype (weak or absent expression of CD10, high expression of CD38, and cytoplasmic immunoglobulin μ-chain), and poor outcome due to early relapse [60,61,62]. The gene discussed is CD38; the disease is acute lymphoblastic leukemia.