In all, 31 studies were excluded as the proportion (%) of COX-2 expression in DCIS was not reported or could not be calculated (n=13), the sample size was below 10 (n=6), the tissue source was used in a larger study (n=9), the study did not state use of monoclonal anti-COX-2 antibodies (n=2) or the study did not report independent evaluation of COX-2 staining by two assessors (n=1). Here, PTGS2 is linked to ductal breast carcinoma in situ.