Lindgren et al. showed a relationship between CA 125 levels and ovarian cancer.22 More recently it was realised that CA 125 levels can also increase in benign serous effusions.08,16,17 Unlike CEA and CA 15-3, CA 125 is secreted from mesothelial cells in patients with PE of benign aetiology.07 For this reason, it can be used to determine the existence of fluid but it does not inform on the aetiology.10,16,17. The gene discussed is CEACAM5; the disease is ovarian cancer.