X-chromosome inactivation pattern of the patient and a control with an X-chromosome inactivation defect. DNA was isolated from the leukocytes and from a buccal smear of the patient (no. 9) as well as from the leukocytes and from cultured fibroblasts of a control with an X-chromosome inactivation defect.31 The DNA samples were incubated with or without the restriction enzyme HpaII and subjected to PCR amplification of the HUMARA locus (see “Methods”). The labeled products were separated by size chromatography and scanned for comparison. Panels A and B, DNA from patient (no. 9) buccal smear; panels C and D, DNA from patient (no. 9) leukocytes; panels E and F, DNA from control fibroblasts; panels G and H, DNA from control leukocytes. The patient shows equal inactivation of X chromosomes in the buccal smear but strongly skewed X-chromosome inactivation in the leukocytes. The control shows strongly skewed X-chromosome inactivation in both fibroblasts and leukocytes. Peaks indicated in gray are the main PCR product peaks; those in white are shorter products due to incorrect polynucleotide synthesis. The calculation of skewing percentage (Table 1) is based on the peak heights of the main PCR products.