The 4-kallikrein (4K) test, which is another PSA-based combinatory marker (total PSA, free PSA, intact PSA and human kallikrein-related peptidase 2 [hK2]) [86], predicted tumour reclassification by upgrading at confirmatory biopsy at 6 months in a cohort of 137 patients on AS who had ISUP GG1 at diagnosis [67]. Here, KLK4 is linked to neoplasm.