ERBB4 and breast cancer: γ-Tocotrienol inhibition of mammary tumor cell growth is mediated by suppression of receptor tyrosine kinase activity of HER3/ErbB3, HER4/ErbB4, and to a lesser extent HER2/ErbB2, but not HER1/ErbB1, and attenuation of receptor downstream pathways that include MAPK, PI3K/Akt, STAT, and NFκB signaling [55]-[57].