The frequency of X chromosome aneuploidy in human female peripheral bl
ood lymphocytes has been reported by several investigators to be signi
ficantly higher than expected based upon chance alone. Studies in our
laboratory showed that 72% of the micronuclei in the peripheral blood
of human females contained the X chromosome. Such a high frequency of
X chromosome loss suggests that some unique mechanism may be responsib
le for this phenomenon. The present study was carried out to test the
hypothesis that the lost or micronucleated chromsome is the inactive a
nd not the active X. Blood samples were obtained from two unrelated fe
males, 36 and 33 years of age, each with a different X; 9 reciprocal t
ranslocation. In each, the normal X chromosome is inactive and the tra
nslocated X is active. Isolated lymphocytes were cultured according to
standard techniques and blocked with cytochalasin B. Using a modified
micronucleus assay, we scored 10,000 binucleated cells from the 36 ye
ar old, while 9,500 binucleated cells were scored from the 33 year old
. The slides were first labeled and the kinetochore status of each mic
ronucleus was determined. This was followed by simultaneous hybridizat
ion with a 2.0 kilobase centromeric X chromosome-specific probe and a
chromosome 9 specific whole chromosome painting probe. All micronuclea
ted cells were relocated and scored for their probe status. A total of
217 micronuclei were scored from the two subjects, of which 96 (44.2%
) contained the X chromosome. Of these 96 micronuclei, 80 (83.3%) cont
ained the inactive X, based on the absence of chromosome 9 material in
the micronucleus. These results support our hypothesis that the inact
ive X chromosome is preferentially included in the micronuclei, and su
ggest that the X chromosome hypoploidy observed at metaphase in aging
women is a related phenomenon.