X-chromosome inactivation, the largest epigenetic event known, involves random silencing of one of the two X chromosomes in the cells of female mammals. In an Advanced Early Publication in Nature Genetics, Fei Xue and colleagues report defects in X inactivation in cells from cloned bovine embryos (NatGenet 2002, DOI:10.1038/ng900).

Xue etal. looked at the allele-specific expression of the X-linked monoamine oxidase type A (MAOA) gene and at the expression of Xist and other X-linked genes in cloned XX calves. They found evidence for aberrant X-chromosome inactivation in deceased clones, and incomplete nuclear reprogramming. They also show that X-chromosome inactivation is paternally imprinted in extra-embryonic tissues of normal cows, but is random in the placentae of deceased clones.

The defective patterns of X inactivation seen in cloned cows are in contrast to the normal X inactivation events reported in cloned mice.