The isolation of yeast centromeres has provided the opportunity to describe the molecular structure of chromosome attachments to the mitotic spindle. Nucleolytic probes of chromatin structure and analysis of genetically altered centromeres have been used to determine the primary sequence and positional requirements for assembly of this chromosomal domain into a functional unit. Structural analysis of the centromere in wild-type and mutant strains provides the first indication of specific proteins associated with centromere DN A in vivo.