A number of unique sequence recombinont DNA clones were isolated from a recombinant DNA library constructed from DNA enriched for chromosome 21 by flow sorting. Of these, five were mapped to chromosome 21 using a somatic cell hybrid. Regional mapping of these probes and of a probe previously assigned to chromosome 21, was carried out with the aid of chromosome 21 rearrangements using both chromosome sorting and a somatic cell hybrid. Three probes were 3hown to be located on either side of the breakpoint 21q21.2. Two of the probes were shown to identify restriction fragment length polymorphisms (RFLPs ) with high rare-allele frequencies (0.46 and 0.43). A Bgl II RFLP revealed the parental origin of non-disjunction in three of ten families with Down's syndrome.
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