AF is the number of acentric fragments and F = 0.5 - 0.5P, where P equals the probability of fragments being included in the traditional nucleus and not forming a micronucleus. One study, which used Giemsa stain to stain nuclear material, established the following criteria for identifying micronuclei: 1) diameter less than 1/3 of the primary nucleus, 2) non-retractility (excludes small stain particles), 3) color the same as or lighter than the main nucleus (excludes large stain particles), 4) location within 3 or 4 nuclear diameters of the main nucleus without touching it, and 5) no more than two associated with one primary nucleus (3 or more micronuclei are likely polymorphs or prorubicytes with nuclear fragments). Assays The micronucleus tests provide important information about a chemical's ability to interfere with chromosome structure and function. For instance, many known human carcinogens test positive in mammalian micronucleus tests. In these tests, organisms are treated with a chemical and the resulting frequency of micronucleated cells is measured. If there is a marked increase in the number of cells with micronuclei, it can be concluded that the chemical induces structural and/or numerical chromosomal damage. Since micronucleus tests must be performed on actively dividing cells, bone marrow stem cells and the erythrocytes they produce through cell divisions are ideal candidates. These cells experience constant, rapid turnover and the lack of a true nucleus in erythrocytes makes micronuclei easily visible under a microscope. Micronucleus assay systems are very economical, require much less skill in scoring that conventional metaphase tests, and are much faster than these conventional tests. Since micronucleus assays reflect chromosomal aberrations reliably and rapidly, they are extremely useful for a quick assessment of chromosomal damage. In particular, the CBMNcyt (cytokinesis-block micronucleus cytome) assay is extremely versatile and is one of the preferred methods to measure the level of chromosomal damage and chromosomal instability in cells. The cytokinesis-block micronucleus (CBMN) assay was first developed to score micronuclei in cells that completed nuclear division by blocking them at the binucleate stage before cytokinesis. It later evolved into the CBMN 'cytome' assay to further explore cell death, cytostasis, and biomarkers of DNA damage. The major drawback of using micronucleus tests is that they cannot determine different types of chromosomal aberrations and can be influenced by the mitotic rate and proportion of cell death, skewing the results. Patterns in formation Multiple studies have found that micronuclei frequency in