ERBB3 and breast cancer: Moreover, other studies found that CARs with 4‐1BB/CD3 endodomains fused to the extracellular domain of heregulin‐1 (HRG1), a natural ligand for HER3/HER4, boosted the killing potential of CAR‐T cells against HER3‐overexpressing SK‐BR‐3 and BT‐474 breast cancer cell lines.