The centromere, a constricted region of the chromosome, is essential for proper chromosome movement in mitosis and meiosis (see Figure 2.6). Centromeres are the binding sites for the kinetochore, to which spindle microtubules attach. In Drosophila, Arabidopsis, and humans, centromeres span hundreds of thousands of base pairs. Most of the centromere is made up of heterochromatin. Surprisingly, there are no specific sequences that are found in all centromeres, which raises the question of what exactly determines where the centromere is. Research suggests that most centromeres are not defined by DNA sequence, but rather by epigenetic changes in chromatin structure. Nucleosomes in the centromeres of most eukaryotes have a variant histone protein called CenH3, which takes the place of the usual H3 histone. The CenH3 histone brings about a change in the nucleosome and chromatin structure, which is believed to promote the formation of the kinetochore and the attachment of spindle microtubules to the chromosome.