The glycophorin A assay was used to estimate the frequency of mutations that accumulate in vivo in somatic cells of persons with Bloom's syndrome (BS). This assay measures the frequency in persons of blood type MN of variant erythrocytes that lack the expression of one allelic form of glycophorin A, presumably due to mutational or recombinational events in erythroid precursor cells. Samples of blood from persons with BS showed dramatic 50- to 100-fold increases in the frequency of variants of three types, those with a hemizygous phenotype, those with a homozygous phenotype, and those with what appears to be partial loss of the expression of one locus. The high frequency of homozygous variants, genetic evidence for altered allelic segregation of a specific biochemical locus, provides evidence for increased somatic crossing-over in vivo in BS. An increased generation of functional hemizygosity and homozygosity in their somatic cells may play an important role in the extreme cancer risk of persons with BS
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