In mammalian cells, 2 broad groups of centromere-interacting proteins have been described: constitutively binding;centromere proteins and passenger, or transiently interacting, proteins (reviewed by Choo, 1997). The constitutive;proteins include CENPA (centromere protein A; MIM 117139), CENPB (MIM 117140), CENPC1 (MIM 117141), and CENPD (MIM;117142). The term passenger proteins encompasses a broad collection of proteins that localize to the centromere;during specific stages of the cell cycle (Earnshaw and Mackay, 1994 (PubMed 8088460)). These include CENPE (MIM;117143); MCAK (MIM 604538); KID (MIM 603213); cytoplasmic dynein (e.g., MIM 600112); CliPs (e.g., MIM 179838); and;CENPF/mitosin (MIM 600236). The inner centromere proteins (INCENPs) (Earnshaw and Cooke, 1991 (PubMed 1860899)), the;initial members of the passenger protein group, display a broad localization along chromosomes in the early stages of;mitosis but gradually become concentrated at centromeres as the cell cycle progresses into mid-metaphase. During;telophase, the proteins are located within the midbody in the intercellular bridge, where they are discarded after;cytokinesis (Cutts et al., 1999 (PubMed 10369859)).