Large intervening non-coding RNAs (lincRNAs) are pervasively transcribed in the genome. Here it is shown that lincRNAs in the HOX genetic loci are dysregulated during breast cancer progression in human cells, and that expression levels of the lincRNA called HOTAIR can predict whether a tumour will metastasize. Moreover, enforced expression of HOTAIR can lead to altered patterns of binding of the PRC2 protein to the genome