The centromere, essential for faithful chromosome segregation during mitosis, has a network of constitutive centromere-associated (CCAN) proteins associating with it during mitosis. So far in vertebrates at least 15 centromere proteins have been identified, which are divided into several subclasses based on functional and biochemical analyses. These provide a platform for the formation of a functional kinetochore during mitosis. CENP-S is one that does not associate with the CENP-H-containing complex but rather interacts with CENP-X to form a stable assembly of outer kinetochore proteins that functions downstream of other components of the CCAN. This complex may directly allow efficient and stable formation of the outer kinetochore on the CCAN platform.