a. The large array D17Z1 on HSA17 is defined by a canonical (wild-type) 16-mer HOR unit. However, D17Z1 is highly polymorphic such that single and multiple monomer deletions produce HOR variants that differ in length by an integral number of monomers. In the general population, HOR variants range from 15-mers to 12-mers. Two major haplotypes (Haplotype I - wildtype; Haplotype II – variant) exist in the population, distinguished by the presence or absence of the 13-mer HOR unit.
b. HSA17 exhibits centromeric epialleles (i.e. multiple sites of centromere assembly) based on the amount of variation within D17Z1. Arrays containing predominantly wild-type 16-mer HORs are preferred locations for centromere assembly, denoted by CENP-A (red circles) and other centromere proteins, and are associated with stable HSA17s. When D17Z1 arrays are composed of more than 70% variant 13-mer HORs, centromere assembly occurs on neighboring D17Z1-B (pink arrows) and chromosome stability is comparable to HSA17s with wild-type arrays. However, centromere assembly occurs on D17Z1 arrays exhibiting intermediate levels of variation (40–60%) but leads to two distinct chromosome phenotypes: stable and unstable. Variant arrays exhibiting the unstable HSA17 phenotype are associated with reduced numbers of centromere proteins.