The plexin family are transmembrane receptors, comprised four subfamilies: plexin-A, B, C, and D. In humans, three members of the plexin-A subfamily have been isolated: plexin-A1, A2, and A3. Plexin A1 is encoded by the PLXNA1 gene.

These receptors are characterized by the presence of a conserved intracellular domain of about 600 amino acids known as the sex and plexins domain. This domain is strikingly conserved across the plexin family and consists of two blocks of high sequence conservation separated by a variable linker. The ectodomains of the plexin-A subfamily possess three repeats of the cysteine cluster/Met-related sequences.

Plexins act as signal-transducing components containing transmembrane domain of six Gly residues constituting a GxGGGGG motif.

Plexins act as functional receptors for members of the semaphorin family. Together they transmit repulsive axon guidance signals for semaphorins inside the axonal growth cone. The plexins act together with their co-receptors, neuropilins. Plexin-A1, in combination with neuropilins exhibits various functions in axon guidance, organogenesis, angiogenesis, tumorigenesis, and immune regulation^1-3.

Plexin-A1 also plays an important role in the developmental stages of chick heart by working as a receptor for one of the transmembrane semaphorins. In addition Plexin-A1 expressed in dendritic cells is involved in T-dendritic cell interactions in the immune system⁴.