Uniparental isodisomy is defined as the inheritance of two copies of t
he same parental chromosome and can result in defects when it produces
homozygosity for a recessive mutation or in the presence of imprintin
g. We describe the detection of a chromosome 6 uniparental isodisomy i
n a 9 year old girl, discovered during a search for an HLA identical s
ib. HLA typing, erythrocyte phenotyping, and genotypes of microsatelli
te polymorphisms were compatible with a paternal isodisomy of chromoso
me 6, with normal biparental origin of the other chromosomes. Paternal
cells were not responsive to the patient's cells in mixed lymphocyte
cultures. This fortuitous detection of a chromosome 6 isodisomy sugges
ts that cases of chromosome 6 UPD may not be deleterious and may there
fore go undetected.