This prospective, in vitro, non-interventional study was designed to accompany an independent proficiency test (IPT; one of three phases of EQA) and descriptively analyze the results of CLDN18.2 IHC testing in gastric cancer tissue samples by laboratories in the DACH region (Germany, Austria, and Switzerland) using LDTs and/or in vitro diagnostic (IVD)-labeled assays independently established in their institutions. This evidence concerns the gene CLDN18 and gastric cancer.