Red blood cells are highly specialised and have the sole job of carrying oxygen around in the blood. They also have a short lifespan. Other cell types have more flexible jobs and need to react to their environment so need to use the information on their genes, stored in the nucleus. Red blood cells develop (or differentiate) from cells with nuclei, so they do need a nucleus in order to exist in the first place. Did you know that bird red blood cells keep their nucleus? Anyone know why???