PSA is the most useful prostate cancer marker. However, its levels are increased also in some non-malignant conditions. In circulation, the majority of PSA is complexed with protease inhibitors, including α1-antichymotrypsin (ACT). The proportion of the PSA-ACT complex is higher in patients with prostate cancer than in controls without cancer. The expression of ACT has been shown to be higher in prostate cancer than in benign prostatic hyperplasia. However, results regarding the extent which PSA forms complexes within the prostate and whether there are differences in complex formation between normal and malignant prostatic tissue are inconsistent and limited.