Autonomous gene expression on the human inactive X chromosome.

Local derepression of the hpt locus on the human inactive X chromosome obtained in human female fibroblast x mouse L cell somatic cell hybrids was not correlated with the presence or absence of any specific human chromosome in the hybrids. Loss of the human active X, in particular, did not result in observable derepression of genes on the inactive X. Introduction of an active X, via a second hybridization of human cells having an active X with hybrid cells containing a locally derepressed X chromosome, did not restore repression of the derepressed hpt allele. The rate of hpt locus derepression in hybrid cells was estimated to be 10(-6) per inactive X chromosome per cell generation.