During mitosis, spindle fibers attach to the centromere via the kinetochore. In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres. Condensed chromosomes consist of two sister chromatids joined at the centromere. Microtubules attach to the midpoint of chromosomes (the centromere) via protein complexes (kinetochores). As a result, each chromosome consists of two sister chromatids held together at the centromere. The two chromatids are joined at the centromere. This allows the detection of antibodies to mitosis-specific antigens, such as centromere antibodies. The term always refers to the chromosome that has an intact centromere. If a chromosome lacks a centromere, it is said acentric. Overexpression of these centromere genes can increase genomic instability in cancers. Chromosomes will also be visible under a microscope and will be connected at the centromere. They are counted from the centromere out toward the telomeres. Notably, overexpression of many centromere genes have been linked to cancer malignant phenotypes. The signal for formation of a regional centromere appears to be epigenetic. Instability of centromere repetitive DNA was recently shown in cancer and aging.