Cosmid libraries have been constructed from DNA of somatic cell hybrid cell lines, each containing a fragment of human chromosome seven and including sequences closely linked to cystic fibrosis (CF). Cosmids containing human DNA as insert were isolated from the library. Three cosmids, when used as probes to total genomic DNA, detected polymorphic loci, each of which was shown to be in strong linkage disequilibrium with CF. Restriction endonuclease digestion of cosmid clones and use of a new, rapid method of chromosome walking based on competitive hybridisation of cosmid inserts has allowed identification of several groups of overlapping cosmids ("contigs") from the vicinity of CF.
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