In general the tropism of AAV is determined by its capsid structure. The capsid of AAV9 preferentially binds to terminal N-linked galactose, a certain sugar structure which is present on the surface of a variety of cells (e.g. as part of glycoproteins). Additional protein(s) which serve as AAV9 receptor(s) need to be present on the cell membrane as well. One candidate is the transmembrane protein KIAA0319L (also termed AAVR). After binding to AAVR the AAVs are taken up by the cell via clathrin-mediated endocytosis togehter with AAVR. The amount of N-linked galactose in the cell membrane together with AAVR and/or other protein(s) that serve as AAV9 receptor(s) determines the amount of AAV9 being taken up by the cell. Skeletal muscle cells seem to present a substantial amount of these molecules on their surface. Keep in mind though, that AAV9 is not 100% specific for skeletal muscle cells and is also able to transduce a large number of different other cell types that also display the relevant AAV9 receptors.