Methylene blue can be used in biology as a stain to indicate whether cells are dead or alive. The stain cannot penetrate live cells however, in dead cells, it can penetrate the cell membrane and the cells will be stained blue. It is also used to study RNA and DNA in gel or under the microscope. Iodine is a useful stain as well. Iodine is used when studying plant cells. It reacts with starch and turns blue-black, because it is a starch indicator. When studying plant cells, iodine can stain not only starch, but also enters the cell wall-cell membrane pores and can facilitate the staining of the nucleus, rendering it more visible under the microscope. Iodine can work as a stain on animal cells as well, causing the cell membrane and nucleus to appear more visible.