PRB3 encodes a glycosylated proline-rich protein (Gl or PRG) that functions as a salivary receptor for the oral pathogen Fusobacterium nucleatum 1. The protein is a major constituent of human parotid saliva and contains tandemly repetitive sequences in exon 3 that encode the proline-rich domain 2. The gene structure consists of four exons spanning approximately 4.0 kb, with significant allelic variation arising from unequal intragenic crossing over that generates different numbers of tandem repeats 3. PRB3 mutations include both length variants from repeat number variation and null mutations; null alleles result from frameshifting insertions that prevent gene expression 4. Functionally, PRB3 null mutations abolish F. nucleatum binding to saliva in vitro, suggesting the protein plays a role in oral bacterial defense 1. Some PRB3 variants produce disulfide-bonded proteins that modify salivary peroxidase activity, contributing to the intraoral antibacterial system 4. Population studies show certain variants like Gl 8 are population-specific (detected in Ashkenazi Jews at ~0.8% frequency but absent in other populations) 4. While genome-wide association studies have identified numerous Alzheimer's disease risk loci, PRB3 does not appear among established AD-associated genes in current large-scale analyses 5.