Micronucleus formation is a hallmark of genotoxicity and micronucleus assay is an important component of genotoxicity screening. The formation of micronuclei serves as an index of genetic damages. Micronuclei induction can result from clastogens or aneugens (agent that induces chromosomal loss mainly through interference with spindle apparatus). The major mechanism responsible for micronuclei induction is double stranded DNA breaks. Micronucleus assay may detects clastogens and aneugens including mitotic delay, apoptosis and chromosomal loss or non-disjunction of chromosomes. Micronuclei are chromatin containing bodies that represents acentric fragments or whole chromosome that were not incorporated into the daughter cell nucleus at cell division. The micronucleus is an excellent biomarker because it is able to detect chromosome breakage or ill functions of mitotic spindle caused by aneugenic mechanisms. The micronucleus index in rodents or human cells is one of the standard cytogenetic tests used in the assessment of genetic toxicity in vivo or ex vivo.