Similarly, patients receiving CD19-directed CAR T cells typically develop neurotoxicity within days of the onset of CRS, often before CAR T cells are detectable in large numbers in the peripheral blood.10–12 This may be explained by the fact that cytokine elevations are highest during the initial expansion of CAR T cells in vivo,10,20 or possibly by a model where the presence of very few CAR T cells is sufficient to trigger a monophasic neurologic injury. This evidence concerns the gene CD19 and congenital rubella syndrome.