PSP94/MSMB: ProductsPSP94 (prostate secretory protein of 94 amino acids; also named beta-MSP) is a secreted, non-glycosylated member of the beta-microseminoprotein family. The 94 amino acid (aa) mature PSP94 contains no classic motifs or domains, but does have ten Cys that are conserved across species. It is expressed in mucoid secretions, but its function is unknown. PSP94 is abundant in prostatic fluid, which is the exclusive source in rodents. Gastric and respiratory secretory epithelia are also significant sources in humans. Human PSP94 circulates bound to a 71 kDa PSP binding protein, possibly disulfide-linked. The seminal fluid protein CRISP-3 can also bind PSP94. PSP94 has been proposed as an alternative to PSA as a serum marker for prostate cancer. When total (bound plus free) PSP94 is considered, its secretion is found to be down-regulated in cancer cells, creating below normal circulating levels. Its size is predicted at 11 kDa, but may appear to be 16 kDa due to anomalous migration. A 61 aa variant, formed by C-terminal proteolysis, is increased in prostate secretions from patients with benign prostatic hyperplasia. A prostate-specific alternate splice form shows a substitution of 41 aa for the C-terminal 78 aa. Mature human PSP94 shares 53%, 46%, 43%, and 42% aa identity with porcine, rat, mouse, and chicken PSP94, respectively. Most of the ten primate sequences available show less than 80% aa identity. PSP94 has been proposed as a species barrier protein due to its low evolutionary conservation and abundance in seminal fluid.