PRB4 encodes a basic glycosylated proline-rich protein (gPRP) expressed exclusively in parotid salivary glands 1. The gene is located on chromosome 12.2 as part of a six-gene PRP cluster and spans approximately 4.0 kb with four exons, where exon 3 contains 63-bp tandem repeats encoding the proline-rich domain 2. PRB4 exists in multiple allelic forms with length polymorphisms arising from variable numbers of tandem repeats in exon 3, generated through intragenic unequal crossing over 3. The gene encodes at least two protein products: the Po protein and the glycoprotein II-1, both absent in PRB4M PO- null alleles carrying a splice-site mutation 4. PRB4-encoded proteins undergo extensive post-translational modifications including N- and O-glycosylation, phosphorylation, and proteolytic processing, generating multiple proteoforms 1. The II-1 glycoprotein serves a biological function in bacterial binding, as its absence alters in vitro binding of Streptococcus sanguis to salivary proteins 4. PRB4 proteins function as salivary biomarkers useful for forensic body fluid identification 5. The high polymorphism of PRB4 contributes significantly to inter-individual variability of the human salivary proteome 6.