MDK and cancer: HER2 is commonly amplified or overexpressed in cancer; however, HER2 gene fusions are rare, present in 0.2–0.3% of solid tumors overall,200,201 and appear to be more common in patients with HER2-positive gastric cancer or breast cancer.201–203HER2 fusions have been detected in various cancers, including gastric, esophageal/gastroesophageal junction, lung, brain, breast, and urothelial cancers, with fusion partners including PGAP3, ZNF207, MDK, NOS2, and MIEN1. 197,200,203 The effectiveness of approved HER2-targeted treatments in HER2 fusion-driven cancer is not well established.