When patients with cancer are given pan-ERBB/HER inhibitors, ERBB4/HER4/ALS19 function is attenuated, but any feedback upregulation of NRG1 should it occur cannot overstimulate ERBB1/2/3 because the pan-ERBB/HER inhibitor diminishes the function of these other ERBBs. Here, NRG1 is linked to cancer.