Micronucleus is also the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. In newly formed red blood cells in humans, these are known as Howell-Jolly bodies. In normal people and many other mammals, which do not have nuclei in their red blood cells, the micronuclei are removed rapidly by the spleen. Hence high frequencies of micronuclei in human peripheral blood indicate a ruptured or absent spleen. In mice, these are not removed, which is the basis for the in vivo micronucleus test.