Studies with fewer than 10 normal breast epithelium, DCIS or MICB samples and 100 invasive breast cancer samples were excluded, only samples which expressed COX-2 at a moderate or higher level (reported by two observers) were considered positive and only studies that used monoclonal anti-COX-2 antibodies were included in an attempt to limit between-study variation. This evidence concerns the gene PTGS2 and ductal breast carcinoma in situ.