AUC for LacdiNAc-PSA (0.851) was significantly greater than that for tPSA (0.559), and the optimum cut-off gave low false positivity (40.7%) and high sensitivity (88.4%), with specific distinction between PC and BPH within the PSA grey zone (4.0–20.0 ng/mL PSA). The gene discussed is KLK3; the disease is pachyonychia congenita.