The TCF3::HLF-rearranged PDX#4 xenograft — an almost universally-fatal, rare ALL subtype typically associated with relapse and death within two years from diagnosis [46, 56, 57] — exhibited substantially decreased extramedullary leukemic growth in response to combination therapy but not total leukemia burden at these subclinical plasma concentrations. The gene discussed is TCF3; the disease is acute lymphoblastic leukemia.