To investigate consistent detection of the basal tissue matrix, we examined prostate-specific antigen (PSA; the protein used in prostate cancer screening26), and found similar intensities among PSA peptides in Samples 1–6 containing 50% prostate cancer tissue, along with some likely false positive observations in Sample 7 containing no prostate tissue (Fig. 2d). This evidence concerns the gene KLK3 and prostate carcinoma.