A comparative analysis of the conserved region of primate alpha satellite DNA and of a human neocentromere is reported. The two types of centromere DNA share structural features, possibly defining mitotically functional human centromere DNA. These features consist of double dyad symmetries of a particular size, as well as short conserved base motifs adjacent to the dyad symmetries. The possible importance of these features is supported by their presence also in the centromeres of yeast. The overall resemblance between primate alpha satellite DNA and yeast centromere DNA is further strengthened by the presence of a second shared structure. This structure consists of a single dyad symmetry incorporating a particular short base sequence. In the mitotically stable human neocentromere, this second symmetry is not found in connection with the double symmetry. However, the yeast homologue of the second symmetry is responsible for stability in meiosis I, rather than for mitotic stability. If the human homolog serves a similar function, it would be expendable, except for reproduction.