A mixed glandular-neuroendocrine gastric carcinoma shows discrete, juxtaposed areas of adenocarcinoma and neuroendocrine carcinoma. In order to gain insight into the genetic events and clonality associated with the dual differentiation of a mixed gastric carcinoma, eight cases (two true composite and six neuroendocrine-dominant carcinomas) were examined by analyzing the genome-wide loss of heterozygosity (LOH). Of the eight mixed gastric carcinomas, one true composite and five neuroendocrine-dominant carcinomas showed a primary LOH that was shared by both the glandular and neuroendocrine components and a secondary LOH or mutations that were restricted to the neuroendocrine components. The glandular components contained mixed cell populations with or without primary LOH events, suggesting that a primary LOH arose during adenocarcinoma progression and that the LOH-positive cell served as a precursor for a neuroendocrine carcinoma. The neuroendocrine components were of a homogeneous population with various genetic alterations such as a LOH, p53 mutations, and microsatellite instability-associated transforming growth factor (TGF)-beta RII mutation. Therefore, most (six of eight cases) mixed glandular-neuroendocrine gastric carcinomas were likely to have sequentially evolved from a glandular precursor to a genetically heterogeneous adenocarcinoma and then to neuroendocrine differentiation. Two components of the other true composite carcinomas were shown to have reciprocally lost different alleles of identical loci on multiple chromosomes, suggesting that, occasionally (one of eight cases), dual differentiation concurrently arises from a single precursor, possibly as a result of non-disjunctional cell division.