We read with great interest the recent report by Garcia-Villoria et all regarding the expression of the HSD17B10 gene from the inactive X chromosome that was published in the European Journal of Human Genetics (Advance online publication, 28 July 2010; doi:10.1038/ejhg.2010.118).It had been reported previously that a"cluster of six genes, including the HSD17B10 (formerly HADH2) gene in Xpl 1.2, escapes X-inactivation.2 Subsequently, Carrel and Willard, in a more detailed study,3 showed that the escape of the HSD17B10 gene from X-inactivation is not complete. The expression of the HSD17B10 gene and the surrounding genes from the inactive X chromosome (Xi) is summarized in Figure 1 (adapted from Ref. Yang et al*).Two female patients heterozygous for HSD10 deficiency were the subjects of this present study1 in which skin fibroblast cultures were examined to determine the inactivation ratio of the normal and mutated X chromosomes. It appears that these studies were performed on cultures originating from a single biopsy from each patient. Mosaicism due to lyonization results in relatively large patches of skin with the same inactivated X chromosome, commonly illustrated by the coloration of calico cats. Thus, an analysis of cells from a single biopsy is probably not adequate to determine the X inactivation ratio. Analysis of a blood sample might be more informative.
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