It consists of the combined aleurone and pericarp. Niacytin is concentrated in the aleurone and germ layers which are removed by milling. Auxin and cytokinin play a role in the earlier stages of aleurone development. This process drives the flavins in the germ and aleurone layers into the endosperm. This causes the seed to lack an aleurone layer. Thick primary cell walls enclose and protect the aleurone cells. The ploidy of the aleurone is (3n) as a result of double fertilization. The maturation of aleurone is promoted by ABA while germination is promoted by GA. The aleurone may also express several pathogen-protective proteins including PR-4. Aleurone proteins can have two different morphological features, homogenous and heterogeneous. The cereal aleurone functions for both storage and digestion. Unlike the cells of the starchy endosperm, aleurone cells remain alive at maturity. The aleurone layer is important for both the developing seed and the mature plant. After completing this function, the aleurone cells in the developing seed undergo apoptosis. In addition, the aleurone tissue contains many protein-storing vacuoles known as protein bodies.