Micronuclei primarily result from acentric chromosome fragments or lagging whole chromosomes that are not included in the daughter nuclei produced by mitosis because they fail to correctly attach to the spindle during the segregation of chromosomes in anaphase. These full chromosomes or chromatid fragments are eventually enclosed by a nuclear membranes and are structurally similar to conventional nuclei, albeit smaller in size. This small nucleus is referred to as a micronucleus. The formation of micronuclei can only be observed in cells undergoing nuclear division and can be clearly seen using cytochalasin B to block cytokinesis to produce a binucleated cells.