FURIN and cancer: Across all cancers, ERBB2 mutations occurred most frequently in the tyrosine kinase domain (62.3%) which included mutations in exon 19 (9.7%), exon 20 (38.3%), exon 21 (11.8%), and others (2.5%), followed by a furin-like cysteine-rich region of the extracellular domain (20.6%) and transmembrane domain (17.1%, Figure 1B).