In the human, males have a pair of sex-chromosomes X and Y in somatic cells whereas females have two X chromosomes, one of which is inactivated (1). Many methods for the male identification have been developed basically with the use of Y chromosome-specific sequences (2-5). However, there are few methods for the positive identification of females and hence samples showing ' Y-negative ' are estimated as female. We here describe a novel method for positively determining female-sex. An X chromosome-specific repetitive sequence at Xq24, designated as DXZ4, has been recently isolated (6). This locus consists of a major cluster of 50-100 copies of a unique 3-kb sequence, which is highly methylated on the active X but is hypomethylated on the inactive X (6). We here report a female-sex typing based on the difference of methylation pattern on the DXZ4 region. This method involves digestion of genomic DNA
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Naito et al. (1993) studied this question.