Determination of maternal and paternal X chromosome activation status is useful in the diagnostic analysis of nonrandom patterns. Skewed patterns of inactivation in female carriers of a number of X-linked recessive disorders and asymptomatic female carriers of some X-linked dominant diseases have been observed. By using the methylation-sensitive restriction enzyme HpaII and the polymerase chain reaction, the methylation status of both the maternal and paternal X chromosome is determined. Methylation of the HpaII restriction endonuclease site in the human androgen-receptor gene, HUMARA, correlates with X-inactivation. In approximately 10% of cases, the testing is uninformative due to the presence of the same allele size on both X chromosomes.