CD4 and breast carcinoma: To determine whether the toxicities observed in the 4T1-tumor bearing mice is specific to the BALB/c background and to test the E3K CAR-T cells in independent models we first assessed CAR-T cell activity in FVB/N mice inoculated with the ER+ HRM1 mouse mammary carcinoma cells.26 BALB/c and FVB/N mice have a similar CD4-heavy CD4:CD8 T cell ratio (figure 2C) and FVB/N and BALB/c E3K CAR-T cells have comparable activity against endosialin+ cells in vitro (online supplemental figures 6 and 10).