– Amyloid β A4 Precursor Protein-Binding Family A Member 3 (APBA3) is an adapter protein that belongs to the X11 family. APBA3 contains 2 PDZ (DHR) domains and 1 PID domain and interacts with the Alzheimer’s disease amyloid precursor protein.. APBA3 is believed to be involved in signal transduction processes. Unlike X11-α and -β which are generally neuronal proteins, APBA3 is widely expressed in all tissues examined with lower levels in brain and testis. It binds to the cytoplasmic domain of amyloid protein (APP) in vivo and may modulate processing of the β-amyloid precursor protein (APP) and hence formation of β-APP.