Given the anticipated direct impact of JAK inhibition upon CAR T cells, we first asked whether ruxolitinib would affect in vivo TSLPRCART fitness and anti-leukemia activity utilizing a known surprisingly ruxolitinib-insensitive IGH::CRLF2 childhood Ph-like ALL PDX model with high TSLPR surface expression (JH331 from Children’s Oncology Group [COG] unique specimen identifier PAMDKS) [15, 19]. The gene discussed is CRLF2; the disease is acute lymphoblastic leukemia.