CEACAM5 and gastric neoplasm: The RT–PCR assay for CEA frequently resulted in false negatives (Marutsuka et al, 2003) because the expression level of the CEA mRNA was heterogeneous in the gastric tumours which exhibited no expression of CEA mRNA (Osaka et al, 2004), or due to the weak expression in the non-cancerous cells, such as the mesothelial cells.