Two α-satellite fragments specific for human chromosome 4 have been cloned and characterized. Under stringent annealing conditions, they hybridized in situ only to the pericentromeric region of chromosome 4, but under nonstringent conditions they hybridized to all chromosomes containing the sequences of α-satellite suprachromosomal family 2 (viz., chromosomes 2, 4, 8, 9, 13, 14, 15, 18, 20, 21 and 22). Southern blot analysis reveals the 3.2-kb higher-order repeated unit which exists in two forms: as a single Msp I fragment or a combination of the 2.6-kb and 0.6-kb Msp I fragments. The two chromosome-4-specific cloned sequences appear to be different parts of this repeated unit. Taken together they constitute about 60% of its length. The primary structure of the higher-order repeated unit is characterized by a dimeric periodicity of the D1-D2 type which is usual to suprachromosomal family 2. At least in one site this regularity is disrupted by monomer deletion leading to the D2-D2 monomeric order. The most likely mechanism of this monomer excision is homologous unequal crossing-over. These sequences may serve as both cytogenetic and restriction-fragment length polymorphism (RFLP) markers for the pericentromeric region of chromosome 4.
