CD19 and leukemia: A high ratio of peak CAR T cell expansion to leukemia burden (> 1) was found to predict longer EFS and OS, suggesting that an optimal ratio of CAR T cells to CD19-positive leukemia cells is more probable in patients with a low leukemia burden than in those with a high leukemia burden, despite the lower CAR T cell expansion in patients with a low leukemia burden [1].