The ROS1 IHC has shown good sensitivity and specificity as a surrogate marker for ROS1 rearrangement in lung adenocarcinomas [90–95], but recent studies showed much lower specificity in gastric adenocarcinomas (3/23) and cholangiocarcinomas (0/72) when compared to NGS assays. The gene discussed is ROS1; the disease is cholangiocarcinoma.