In the case of breast cancer patients mutations within this exon, they were reported to increase the HER2 catalytic activity [81,82], being the L755 (e.g., L755S and L755P), associated with resistance to lapatinib therapy [58,81,83,84], and L869R, together with T798I, promoting HER2 signaling and oncogenic growth, associated to neratinib acquired resistance [22,58]. This evidence concerns the gene ERBB2 and breast cancer.