attributed this statistic to the possible mitotic instability of ring chromosomes. Centromeres are conventionally defined by dark-staining heterochromatin which consists of the primary constriction site—this convention exists because heterochromatin is usually found flanking the centromere. With this in mind, centromeric heterochromatin was found on the bisatellited and ring markers, but not on the mar del(10) marker. However, mar del(10) was still able to stably segregate in vivo and in vitro which implied the presence of a functional centromere and kinetochore. There has been an assumption that centromeric heterochromatin is just as important as the kinetochore in segregating and stabilizing chromosomes, because heterochromatin is associated with protein recruitment and has the defining ability to silence gene expression. However, this notion is challenged by the observation in mar del (10). A decade after the initial observation of neocentromeres, 60 more cases of human neocentromeres from across the genome, not just chromosome 10, were documented by 2002. These cases were also typically observed in children with developmental delay or congenital abnormalities. By 2012, more than 90 cases of human neocentromeres across 20 different chromosomes have been described. Formation Studies suggest that neocentromeres are ultimately formed from epigenetic processes, rather than from changes in DNA sequence. There is a general consensus that neocentromeres result from an attempt to fix chromosomes that lack a conventional centromere, through chromosome rearrangements. The most common type of rearrangement leading to a neocentromere is an inverted duplication, categorized as Class I. The resulting marker chromosome consists of two copies of the chromosome segment. Each of the two copies is a mirror image to the chromosome segment. The neocentromere forms at an interstitial site, between the break point and one of the telomeres. Despite having identical sequences in two halves of the chromosome, the neocentromere is only formed once. In some cases, segregation of these chromosomes result in partial trisomy, and at other times, a partial tetrasomy. In partial tetrasomy, the karyotype appears normal except for the marker chromosome. Mechanisms of formation of neocentromeres are still unclear, but a few have been proposed. It is strongly speculated that neocentromeres form during mitosis or meiosis. For Class I, the proposed mechanism is that chromatid breakage occurs during mitosis, resulting in a chromosome fragment. That acentric chromosome fragment may segregate with the intact chromatid and result in partial tetrasomy; or, it may segregate with the complementary broken chromatid and result in partial