NRG1 and breast cancer: Menendez et al (2006) found Herceptin to inhibit certain breast cancer cells that do not overexpress ErbB2 but do overexpress heregulin. Although MDA-MB-231 does not overexpress ErbB2 or Grb7, it does overexpress heregulin and promotes tumourigenicity and metastasis of breast cancer cells (Aguilar and Slamon, 2001).