Amoureux et al. BMC Cancer 2010, 10:91 http://www.biomedcentral.com/1471-2407/10/91 Page 7 of 12 Figure 3 PSA-NCAM is negatively correlated to overall (OS) and disease free (DFS) survival. Survival curves of patients from the two groups: PSA-NCAM>10 pg/μg of protein (n = 39 for OS; n = 38 for DFS) and PSA/NCAM ≤ 10 pg/μg of protein (n = 17 for OS; n = 16 for DFS). (A) OS: Kaplan-Meier curves indicate for each time point the proportion of patients still alive at that time. (B) DFS: Kaplan-Meier curves indicate for each time point the proportion of patients that did not yet have a relapse at that time. (+): censored patients. that contained 19.5 pg PSA-NCAM/μg of protein as measured by ELISA. Whereas C6 cells did not express olig2, C6-PSA-NCAM displayed strong olig2 expression indicating that in these cells, forced expression of PSA is sufficient to induce olig2 as shown by western blot analysis in Figure 4C. Discussion In the current study, we show that PSA-NCAM is a prognosis factor in GBM, by using a new ELISA assay for PSA-NCAM. This test allows quantifying accurately, specifically, and with high sensitivity PSA-NCAM on small amounts of biopsy materials. The results obtained were well correlated to immunohistochemistry analysis, showing the robustness of the assay. However, some samples displayed different levels of PSA, depending on the method of measurement. The non-linear scoring by IHC and the possible heterogeneity of the tumor may partly explain the discrepancy between the two methods. High PSA values by ELISA could therefore represent false positives (type I error) as their IHC score was 0 in the PSAELISA >10 pg/μg of protein group (12 cases), or false negatives (type II error) as their IHC value was much higher than the median IHC score in the