PSP94 (prostatic secretory protein-94), also known as β-microseminoprotein or prostatic inhibin-like protein, is a small, nonglycosylated peptide consisting of 94 amino acids with molecular mass 10.7 kDa, and is one of the major secretory proteins of the prostate glands. PSP94 is synthesized as a preprotein of 114 amino acid residues, from which a 20-residue signal peptide is cleaved off to form the mature protein. PSP94 along with PSA (Prostate-specific Antigen) and PAP (Prostate Acid Phosphatase) are the three most abundant proteins in seminal fluid. As with other prostate-secreted proteins, PSP94 can leak into the blood upon benign or malignant prostate epithelial disruption and can be measured within serum. PSP94 is not solely synthethized by the prostate epithelium, as the protein can also be detected in nonreproductive organs such as in the respiratory and gastrointestinal tracts, where the gastric mucosa particularly shows high expression. Accordingly, PSP94 can be measured in serum of both men and women, but the levels in serum from women were found to be around two-thirds of those measured in men. PSP94 forms high-affinity complex with two related Cys-rich proteins: PSP94-binding protein in blood plasma and cysteine-rich secretory protein 3 (CRISP-3) in semen. PSP94 has postulated systemic function including growth regulation and induction of apoptosis in prostate cancer cells in vitro and in vivo, and regulation of calcium levels during hypercalcemia secondary to malignancy. Several studies have demonstrated a progressive decrease in PSP94 expression as prostate cancer progresses from a hormone-dependent to a hormone-independent state with complete lack of PSP94 production in highly advanced metastatic prostate cancer. This differential expression could make PSP94 a prognostic clinical marker for prostate cancer and could help distinguish patients with aggressive forms of prostate cancer. In a recent study demonstrated a close correlation between PSP94 in serum and seminal plasma, which supports the potential use of PSP94 as a serum marker of prostate secretory function as well.