Protein amnionless is is a type I transmembrane protein. It is thought to modulate bone morphogenetic protein (BMP) receptor function by serving as an accessory or coreceptor, and thus facilitates or hinders BMP binding. It is known that the mouse AMN gene is expressed in the extraembryonic visceral endoderm layer during gastrulation, but it is found to be mutated in amnionless mouse.

The encoded protein has sequence similarity to short gastrulation (Sog) and procollagen IIA proteins in Drosophila.Long isoforms are highly expressed in small intestine, colon and kidney (renal proximal tubule epithelial cells). Shorter isoforms are detected at lower levels in testis, thymus and peripheral blood leukocytes.