Identification and chromosomal mapping of a duplicated X-specific human sequence.

We report the identification of a sequence (CX-10b-1-2; 0.55 kb Hind m/Alu I fragment) which is duplicated on the human X chromosome. On Southern analysis at high stringency A), while Pst I gave a single band, Xmn I and Pvu II three bands, and Bst N I and Hae in at least four bands. Multiple bands may be due to restriction sites within the probe, or two different clusters of the probe sequence. Two bands were seen with Hpa II digests of female but not male DNA, suggesting detection of a differential methylation pattern (Am J Hum Genet 42: 267-270; 1980). The bands ranged in size from <1 kb to >10 kb. Pulsed-field gel electrophoresis (PFGE) of genomic DNA digested to completion with Sfi I and BssH II similarly detected two distinct bands (440 kb and 350 kb for Sfi I, and 550 kb and 460 kb for BssH n)(Fig IB). Dosage analysis (not shown) revealed a hybridization ratio of 1:2 in male versus female DNA, suggesting localisation of the sequence on the X chromosome. In situ hybridization confirmed this and mapped the sequence to the Xq 21.3-q22 region (Fig 2). These results indicate that the CX-10b-1-2 sequence is duplicated at least once on the X chromosome. The distance between the duplicated segments may be greater than 300-500 kb (as seen from pulsed-field experiments) but less than that resolvable by in situ experiments. This sequence should be useful for structural and linkage analysis of the q21-q22 region of the human X chromosome.