Plexin A3 (also known as SEX) is a 220 kDa member of the Plexin A subfamily of semaphorin receptors. Plexin A3 interacts with neuropilins 1 and 2 and binds class 3 semaphorins.

Mature mouse Plexin A3 is predicted to be an 1853 amino acid (aa) type I transmembrane glycoprotein that contains a 1202 aa extracellular domain (ECD) and a 629 aa cytoplasmic region. The ECD contains one semaphorin domain (aa 34 - 473), three plexin-semaphorin-integrin (PSI) domains (aa 492 - 830) and four lg-like plexin-transcription factor (IPT) repeats (aa 841 - 1209). Over aa 35 - 150, mouse Plexin A3 is 99% and 97% aa identical to rat and human Plexin A3, respectively.