Among other functions, the Notch signaling pathway contributes to the development of somites in animals. It involves a cell signaling mechanism with a wide range of functions, including cellular differentiation, and the formation of the embryonic structures (embryogenesis). All multicellular animals use Notch signaling, which is involved in the development, maintenance, and regeneration of a range of tissues. The Notch signaling pathways spans two cells, and consists of receptor proteins, which cross one cell's membrane and interacts with proteins on adjacent cells, called ligands. The physical interaction of receptors and ligands directs the genetic response of the first cell to produce proteins that define the type of cell it will become. One of the earliest discovered roles of the Notch signaling pathway in vertebrates is in somite formation (somitogenesis). Somitogenesis is the formation of somites, which are sphere-like structures in early vertebrate embryos that are the first visible signs of segmentation. Somites then help to define many tissues and features of the adult animal's body. The Notch signaling pathway plays at least two distinct roles during somitogenesis: the first is maintenance of an oscillating protein gradient, called the segmental clock, and the second is establishing the polarity of somites. Mutations to genes in the Notch pathway can result in birth defects characterized by abnormal development of bones of the spine and ribs, like spondylocostal dysostosis. Additionally, dysfunction in the pathway linked to cancer progression, HIV-related complications, and Alzheimer´s disease, among other disorders.