Human LRRTM3 (leucine-rich repeat transmembrane neuronal 3) is a 63kD (predicted) type I transmembrane glycoprotein that is one of four LRRTM family members within the LRR protein superfamily (1). The LRRTM3 cDNA encodes 581aa, including a 389aa extracellular region that shares 98% aa identity with mouse and rat LRRTM3. Isoform 2 lacks cytoplasmic aa514-581. LRRTMs in mouse show distinct expression patterns during formation of the central nervous system (CNS) and expression persists in adult brain. LRRTM3 promotes processing of amyloid precursor protein by BACE1, and may be mutated in late-onset Alzheimer’s disease.