There are a pair of approved CAR T drugs, Yescarta (axicabtagene ciloleucel) from Gilead Sciences Inc. and Kymriah (tisagenlecleucel) from Novartis AG, that have been available since 2017 for a few hematological cancers including some lymphomas and leukemias. But little is known about how these engineered chimeric antigen receptor T cells that both target CD19, an antigen prevalent in the cells of many B-cell malignancies, move through the body and proliferate after they are first removed, altered, expanded in number and, finally, returned to a patient's body.